Fuzzy Nilpotent Lie Algebras: Bases, Isomorphisms, and a Measure of Nilpotency

1. Introduction

The interplay between fuzzy set theory and algebraic structures has been a fertile area of research since Zadeh’s foundational work [1]. The idea of endowing classical algebraic objects with a graded membership function — rather than a sharp, crisp boundary — opens up a richer landscape in which structural properties can be studied not just as binary predicates, but as matters of degree. After Zadeh, Rosenfeld started researching how the fuzzy set theory could be used in abstract algebra, and wrote about this in a well-known work [2]. He was inspired by a paper [3], in which appears the first occurrence of fuzzy set theory used in topological spaces. For more about the history of this topic the reader is referred to [4]. Lie algebra is a well-known algebraic structure that is used a lot for its complex bracket operations and its links to geometry and physics. It is a great subject to use in this type of fuzzy enrichment.

The concept of a fuzzy Lie subalgebra, first discussed in [5], has since been extensively investigated, with a particularly detailed analysis being offered in [6]. The majority of the current research concentrates on the definitional and categorical dimensions of fuzzy Lie subalgebras, encompassing their substructures, homomorphisms, and ideals. The question of nilpotency, however, has received comparatively little attention in the fuzzy setting, despite being one of the most central notions in classical Lie theory.

The present paper grows out of two conference contributions [7,8]. The first [7] laid the groundwork by investigating the foundational properties of fuzzy Lie algebras as fuzzy vector spaces, with particular attention to the notion of a fuzzy basis and the conditions under which a classical basis retains its fuzzy character. The second [8] introduced a definition of nilpotency tailored to the fuzzy setting and undertook a comparative analysis with both the classical notion and existing alternatives in the literature. Here, we consolidate and significantly extend those results, providing a more complete and unified treatment.

Specifically, we introduce a definition of nilpotency for fuzzy Lie subalgebras via an ascending central series adapted to the fuzzy context inspired by the one previosly introduced in [9] for fuzzy groups. We also show that while nilpotency of the underlying crisp Lie algebra implies fuzzy nilpotency, the converse fails — a phenomenon that is both natural and, we believe, geometrically meaningful. To capture this asymmetry, we propose a numerical invariant $N\left(\mu \right)\in [0,1]$, which we call the fuzzy nilpotency measure of a fuzzy Lie subalgebra $\mu$. This measure equals 1 precisely when $\mu$ is fuzzy nilpotent, and takes smaller values the further $\mu$ is from nilpotency.

We also study isomorphisms between fuzzy Lie subalgebras, proposing a definition based on the transfer principle through t-cut sets. We compare this with the categorical notion developed in [10], and we prove that isomorphic fuzzy Lie subalgebras necessarily share the same nilpotency measure. Finally, we interpret N as a fuzzy measure in the sense of Sugeno, and establish some of its basic properties, including subadditivity.

The paper is organised as follows. Section 2 recalls the necessary background on Lie algebras and fuzzy Lie subalgebras, including fuzzy bases and t-cut sets. In Section 3, we study isomorphisms of fuzzy Lie subalgebras and their connection with flags of subalgebras. Section 4 introduces nilpotent fuzzy Lie subalgebras and the fuzzy nilpotency measure N, along with its basic properties.

2. Preliminaries

A vector space $\mathfrak{g}$ over a field $\mathbb{F}$ is a Lie algebra if it is equipped with a bilinear map $[·,·]:\mathfrak{g}\times \mathfrak{g}\to \mathfrak{g}$, the Lie bracket, satisfying the following properties:

$$\begin{array}{cc}\hfill & [x,x]=0_{\mathfrak{g}}\mathrm{for}\mathrm{all}x\in \mathfrak{g},\hfill \end{array}$$

(1)

$$\begin{array}{cc}\hfill & [x,[y,z]]+[y,[z,x]]+[z,[x,y]]=0_{\mathfrak{g}},\hfill \end{array}$$

(2)

for every $x,y,z\in \mathfrak{g}$.

This study focuses on finite-dimensional Lie algebras, meaning those with a finite basis as vector spaces. As illustrated by Equation (1), this bilinear map is alternating, and it is crucial to recognise that this implies the map is skew-symmetric. Indeed, we have

$$[x+y,x+y]=0_{\mathfrak{g}}.$$

Thus, since the Lie bracket is bilinear, the previous equation and Equation (1) imply that

$$[x,y]+[y,x]=0_{\mathfrak{g}},$$

and then

$$[x,y]=-[y,x].$$

(3)

If the field $\mathbb{F}$ has a characteristic that is not 2, then the last condition on the Lie bracket is equivalent to requiring that it is alternating, since

$$[x,x]=-[x,x]\Rightarrow 2[x,x]=0_{\mathfrak{g}}\Rightarrow [x,x]=0_{\mathfrak{g}}.$$

Given the assumptions on the underlying field, equation (3) can replace the initial formulation. Throughout this work, we consider Lie brackets defined over real vector spaces, that is, over the field $\mathbb{R}$. Unless stated otherwise, the Lie bracket is assumed to be skew-symmetric by definition.

The identity in Equation (2), known as the Jacobi identity, plays a central role in Lie algebra theory. It ensures the coherent behaviour of the Lie bracket operation. A detailed treatment of this identity is beyond the scope of this paper; for further reading, see [11,12].

As is common in abstract algebra, once a structure is defined, its substructures are also introduced. These are subsets that themselves satisfy the axioms of the original structure (e.g., subgroups, subrings, linear subspaces). Lie subalgebras follow this pattern: they are vector subspaces of a Lie algebra that also form Lie algebras under the same bracket operation.

The notion of a fuzzy Lie subalgebra was first introduced by Yehia in [5]. The reader will find the majority of the definitions of fuzzy Lie algebra that are to be recalled in the same paper. For our purposes, we adopt the formalism and notation from [6] Definitions 1.16–1.17.

Definition 1. 

Let $\mathfrak{g}$ be a Lie algebra over a field $\mathbb{F}$. A fuzzy set $\mu :\mathfrak{g}\to [0,1]$ is called a fuzzy Lie subalgebra of $\mathfrak{g}$ over $\mathbb{F}$ if

1. 

$\mu (x+y)\ge min\left\{\mu \right(x),\mu (y\left)\right\}$,

2. 

$\mu \left(ax\right)\ge \mu \left(x\right)$,

3. 

$\mu \left(\right[x,y\left]\right)\ge min\left\{\mu \right(x),\mu (y\left)\right\}$,

for all $x,y\in \mathfrak{g}$ and $a\in \mathbb{F}$.

As a consequence of the second condition, we obtain $\mu (-x)\ge \mu \left(x\right)$ and $\mu \left(0\right)\ge \mu \left(x\right)$ for all $x\in \mathfrak{g}$.

The first two conditions in Definition 1 characterise the fuzzy closure of vector addition and scalar multiplication, respectively. Together, they define a fuzzy subspace. The third condition is particularly significant, ensuring the fuzzy closure under the Lie bracket. From now on, we write $\mu \le \mathfrak{g}$ to indicate that $\mu$ is a fuzzy Lie subalgebra of $\mathfrak{g}$. Here, we recall some basic properties about fuzzy Lie subalgebras that will be utilised in our subsequent investigations.

Proposition 1. 

Let μ be a fuzzy subspace of a vector space V. Then, for all $x,y\in V$, we have

1. 

$\mu \left(x\right)=\mu (-x)$.

2. 

If $\mu (x-y)=\mu \left(0\right)$, then $\mu \left(x\right)=\mu \left(y\right)$.

3. 

If $\mu \left(x\right)<\mu \left(y\right)$, then $\mu (x-y)=\mu \left(x\right)=\mu (y-x)$.

We now recall that, if $\mu :\mathfrak{g}\to [0,1]$ is a fuzzy subset of a Lie algebra $\mathfrak{g}$ over a field $\mathbb{F}$, the (crisp) set

$$U(\mu ,t)=\{x\in \mathfrak{g}\mid \mu (x)\ge t\}$$

is the t-cut set of $\mu$ for every $t\in [0,1]$.

The final part of this section introduces definitions and results that are essential for our study. We begin with a key concept from [13].

Definition 2. 

Let V be a fuzzy vector space over a field $\mathbb{F}$. A set of vectors $\mathcal{B}=\left\{e_1,\dots ,e_n\right\}$ is said to be fuzzy linearly independent if

1. 

$\left\{e_1,\dots ,e_n\right\}$ is linearly independent,

2. 

$\mu \left({\sum }_{i=1}^n{a}_i{e}_i\right)={\bigwedge }_{i=1}^n\mu \left(a_i{e}_i\right)$,

for all $a_i\in \mathbb{F}$, $1\le i\le n$.

Definition 3 

([13] Definition 4.1). A fuzzy basis of a fuzzy vector space V is a fuzzy linearly independent set that spans V.

In the classical setting, a basis of a vector space is a maximal set of linearly independent vectors that spans the space. Analogously, a fuzzy basis is a spanning set of fuzzy linearly independent vectors.

Any finite basis of a vector space V is trivially a fuzzy basis of V. Indeed, if $\left\{e_1,\dots ,e_n\right\}$ is a crisp basis, then for any scalars $a_i\in \mathbb{F}$, we have $a_i{e}_i\in V$ and ${\sum }_{i=1}^n{a}_i{e}_i\in V$. Therefore, $\mu \left(a_i{e}_i\right)=1$ and $\mu \left({\sum }_{i=1}^n{a}_i{e}_i\right)=1$, satisfying condition (2) in Definition 2.

We now recall some properties of fuzzy Lie algebras from [7].

Proposition 2 

([7] Proposition 2). Let $\mathfrak{g}$ be a real Lie algebra and let $\mu \le \mathfrak{g}$. Then

1. 

$\mu \left(0_{\mathfrak{g}}\right)=sup\left\{\mu \left(x\right)\mid x\in \mathfrak{g}\right\}$.

2. 

$\mu \left(ax\right)=\mu \left(x\right)$, for all $a\in {\mathbb{R}}^{*}$ and $x\in \mathfrak{g}$.

3. 

If $\mu \left(x\right)\ne \mu \left(y\right)$ for $x,y\in \mathfrak{g}$, then

$$\mu (x+y)=\mu \left(x\right)\wedge \mu \left(y\right).$$

We now extend the concept of fuzzy bases to Lie algebras. In classical Lie theory, the basis of a Lie algebra is simply a basis of the underlying vector space. The same idea applies in the fuzzy context.

In this fuzzy setting, the notion of ideal of a Lie algebra, i.e. a subalgebra I of $\mathfrak{g}$ such that $[I,\mathfrak{g}]\subseteq I$, has a fuzzy counterpart.

Definition 4. 

A fuzzy set $\mu :\mathfrak{g}\to [0,1]$ is called a fuzzy Lie ideal of $\mathfrak{g}$ if

1. 

$\mu (x+y)\ge min\left\{\mu \right(x),\mu (y\left)\right\},$

2. 

$\mu \left(\alpha x\right)\ge \mu \left(y\right),$

3. 

$\mu \left(\right[x,y\left]\right)\ge \mu \left(x\right)$

for alll $x,y\in \mathfrak{g}$, $\alpha \in \mathbb{F}.$

After this definition, we have to remember this important tool in the theory of fuzzy Lie algebra.

Theorem 1. 

A fuzzy set μ of a Lie algebra $\mathfrak{g}$ is a fuzzy Lie subalgebra of $\mathfrak{g}$ if and only if each nonempty set $U(\mu ,t)$ is a Lie subalgebra of $\mathfrak{g}.$

More precisely, this result extends to a broad class of algebras. Statements of the form “a subset A has property P” are classified by Kondo and Dudek [14] as type 0. They established a transfer principle that allows one to lift such properties from classical algebraic structures to their fuzzy counterparts, and vice versa. As they observed, this principle also suggests that genuinely new results in fuzzy algebra should go beyond what can be directly inferred from it. For further details, we refer the reader to the paper cited above and the references therein.

We conclude this section with a result from [7] that provides a practical criterion for deciding when, under a particular choice of the membership function, a basis of a Lie algebra is also a fuzzy basis. Here we recall that a set $\mathcal{B}=\{e_1,\dots ,e_n\}$ is a fuzzy basis for a real vector space V if the elements of $\mathcal{B}$ are fuzzy linearly independent elements, i.e.

$$\mu \left(\sum _{i=1}^n{a}_i{e}_i\right)=\underset{i=1}{\overset{n}{\bigwedge }}\mu \left(a_i{e}_i\right)$$

for all $a_1,\dots ,a_n\in \mathbb{R}$.

Theorem 2. 

Let $\mu \le L$ and $\mathcal{B}=\left\{e_1,\dots ,e_n\right\}$ a basis of L with $\mu \left(e_1\right)=a$ and $\mu \left(e_i\right)=b$, with $1<i\le n$ and $a\ge b$. The set $\mathcal{B}$ is a fuzzy basis of μ if and only if

$$\mu \left(x\right)=\underset{i\in \left\{1,\dots ,n\right\}}{min}\mu \left(e_i\right)=b,$$

(4)

for every $x\in L\setminus \mathrm{span}\left\{e_1\right\}$.

3. Isomorphisms of Fuzzy Lie Subalgebras

In this section, we propose a definition of isomorphism between fuzzy Lie subalgebras based on the transfer principle via t-cut sets, and we compare it with the categorical notion introduced in [10].

For our purposes, we only need the notion of a filtration on an object in a category and, more importantly, that of a morphism of filtered objects, which will serve as the foundation for our definition of isomorphism between fuzzy Lie subalgebras. These concepts were originally introduced by Deligne [15] in the setting of abelian categories, but since we do not require any of the additional structure that such categories provide, the definitions make sense in any category, and in particular in the category $\mathsf{Lie}$ of Lie algebras.

Let $\mathsf{C}$ be a category and let A be an object of $\mathsf{C}$. A filtration $\mathcal{F}$ on A is a family of subobjects ${\left({\mathcal{F}}^{n}A\right)}_{n\in \mathbb{Z}}$ such that

$$n\le m\Rightarrow {\mathcal{F}}^{n}A\subset {\mathcal{F}}^{m}A.$$

The pair $(A,\mathcal{F})$ is called a filtered object, and the filtration is said to be finite if there exist integers $n,m\in \mathbb{Z}$ such that ${\mathcal{F}}^{n}A=A$ and ${\mathcal{F}}^{m}A=0$. A morphism of filtered objects $f:(A,\mathcal{F})\to (B,\mathcal{F})$ is a morphism $f:A\to B$ in $\mathsf{C}$ such that

$$f\left({\mathcal{F}}^{n}A\right)\subset {\mathcal{F}}^{n}B\mathrm{for}\mathrm{all}n\in \mathbb{Z}.$$

Let now $\mathfrak{g}$ be a Lie algebra and let $\mu$ be a fuzzy Lie subalgebra of $\mathfrak{g}$. Clearly, if $t_1<t_2$, then $U(\mu ,t_1)\supset U(\mu ,t_2)$. Hence, given a partition of the interval $[0,1]$, the corresponding family of t-cut sets defines an increasing filtration of $\mathfrak{g}$ (for more results on t-cuts set and partitions, we refer the reader to [7]). Under these assumptions, and in view of the definitions above, we introduce the notion of isomorphism of fuzzy Lie subalgebras as follows.

Definition 5. 

Let $\mu ,\nu$ be two fuzzy Lie subalgebras of $\mathfrak{g}$ and $\mathfrak{h}$, respectively. We say that μ is isomorphic to ν, denoted by $\mu \cong \nu$, if for every $t\in [0,1]$ there exists an isomorphism of Lie algebras $f_t:U(\mu ,t)\to U(\nu ,t)$.

In the previous definition, it follows straightforwardly that the Lie algebras $\mathfrak{g}$ and $\mathfrak{h}$ are isomorphic via $f_0$.

Corollary 1. 

If $\mu \cong \nu$, then $\mathfrak{g}\cong \mathfrak{h}$.

Thanks to the previous result, from now on we may, without loss of generality, restrict our attention to isomorphisms of fuzzy Lie subalgebras of the same Lie algebra.

Example 1. 

Regarding Definition 5, one might expect that it is sufficient to consider an automorphism f of $\mathfrak{g}$ and define $f_t:=f_{\left|U\right(\mu ,t)}$ for every $t\in [0,1]$. However, this is not the case.

Indeed, let $\mathfrak{g}=\langle E_1,E_2,E_3,E_4\rangle$ be the 4-dimensional Lie algebra generated by the matrices

$$E_1=\left(\begin{array}{cccc}0& 0& 0& 0\\ 1& 0& 0& 0\\ 0& 0& 0& 0\\ 0& 0& 0& 0\end{array}\right),E_2=\left(\begin{array}{cccc}0& 0& 0& 0\\ 0& -1& 0& 0\\ 0& 0& 0& 0\\ 0& 0& 0& 0\end{array}\right),E_3=\left(\begin{array}{cccc}0& 0& 0& 0\\ 0& 0& 0& 0\\ 0& 0& 1& 0\\ 0& 0& 0& 0\end{array}\right),E_4=\left(\begin{array}{cccc}0& 0& 0& 0\\ 0& 0& 0& 0\\ 0& 0& 0& 0\\ 0& 0& 0& 1\end{array}\right).$$

The only nontrivial bracket between the generators is $[E_1,E_2]=E_1$. Consider the ideal $I=\langle E_1,E_2\rangle$. This ideal is not characteristic (see [16]), meaning that there exists an automorphism f of $\mathfrak{g}$ such that $f\left(I\right)\ne I$. For instance, define f by $f\left(E_2\right)=E_2+E_3$ and $f\left(E_i\right)=E_i$ for $i\in \{1,3,4\}$. Then $f\left(I\right)\ne I$. Now let μ be a fuzzy Lie subalgebra of $\mathfrak{g}$ defined by $\mu \left(0_{\mathfrak{g}}\right)=0.9$, $\mu \left(E_1\right)=\mu \left(E_2\right)=0.8$, and $\mu \left(E_3\right)=\mu \left(E_4\right)=0.6$, and let $\nu =\mu$. Then, for $t=0.8$, we have $U(\mu ,0.8)=I$, but $f\left(U\right(\mu ,0.8\left)\right)=f\left(I\right)\ne I=U(\nu ,0.8)$.

Proposition 3. 

Let $\mu ,\nu$ be two fuzzy Lie subalgebras of $\mathfrak{g}$ and let f be an automorphism of $\mathfrak{g}$ such that $\mu \left(x\right)=\nu \left(f\right(x\left)\right)$. Then, μ is isomorphic to $\nu .$

Proof. 

We have $f\left(U\right(\mu ,t\left)\right)\subseteq U(\nu ,t)$ for every $t\in [0,1].$ Indeed, suppose that $x\in U(\mu ,t).$ Then $\mu \left(x\right)\ge t$ and hence

$$\nu \left(f\right(x\left)\right)=\mu \left(x\right)\ge t.$$

Moreover, let $y\in U(\nu ,t).$ Then $f^{-1}\left(y\right)\in U(\mu ,t)$, indeed

$$\mu \left(f^{-1}\left(y\right)\right)=\nu \left(f\left(f^{-1}\left(y\right)\right)\right)=\nu \left(y\right)\ge t$$

and $f\left(f^{-1}\left(y\right)\right)=y.$ □

We recall now the notion of isomorphism between fuzzy Lie subalgebras of a Lie algebra $\mathfrak{g}$ presented in [10]. Let ${\mathsf{Lie}}^F$ be the category whose objects are the pairs $(\mathfrak{g},\mu )$, where $\mathfrak{g}$ is a Lie algebra and $\mu$ is a fuzzy subalgebra of $\mathfrak{g}$, and whose morphisms $f:(\mathfrak{g},\mu )\to (\mathfrak{h},\nu )$ are homomorphisms of Lie algebras $f:\mathfrak{g}\to \mathfrak{h}$ such that $\mu \left(x\right)\le \nu \left(f\right(x\left)\right).$ We observe that an isomorphism $f:(\mathfrak{g},h)\to (\mathfrak{h},\nu )$ in ${Lie}^F$ is an isomorphism of Lie algebras $f:\mathfrak{g}\to \mathfrak{h}$ such that $\mu \left(x\right)=\nu \left(f\right(x\left)\right)$ and $\nu \left(y\right)=\mu \left(f^{-1}\left(y\right)\right)$ for any $x\in X$ and $y\in Y.$

Proposition 4. 

Let $f:\mathfrak{g}\to \mathfrak{h}$ be an isomorphism of Lie algebras and let μ be a fuzzy Lie subalgebra of $\mathfrak{g}$. Then there exists a fuzzy Lie subalgebra ν of $\mathfrak{h}$ such that $(\mathfrak{g},\mu )$ and $(\mathfrak{h},\nu )$ are isomorphic in ${\mathsf{Lie}}^F$.

Proof. 

Since every $y\in \mathfrak{h}$ is given by $y=f\left(x\right)$, with $x\in \mathfrak{g}$, hence the statement is proven by defining $\nu \left(y\right):=\mu \left(x\right)$. □

Let $\mathfrak{g}$ be a Lie algebra and let $\mathcal{B}$ be a basis of $\mathfrak{g}$. Now, consider ${\mathcal{B}}^{\prime }$ another basis of $\mathfrak{g}$. Thus, by Theorem 2 and Proposition 4, it follows that ${\mathcal{B}}^{\prime }$ is a fuzzy basis of $\mathfrak{g}$ as in Theorem 2.

Definition 6. 

Let $\mathfrak{g}$ be a Lie algebra. We say that $\mathfrak{g}$ has a flag of subalgebras if there is a chain

$$0={\mathfrak{g}}_n<{\mathfrak{g}}_{n-1}<\dots <{\mathfrak{g}}_0=\mathfrak{g}$$

where ${\mathfrak{g}}_i$ is a $(n-i)$-dimensional subalgebra of $\mathfrak{g}$ for $0\le i\le n.$

In other words, a flag of subalgebras is a finite filtered object in the category $\mathsf{Lie}$ of Lie algebras. A huge class of Lie algebras that admit a flag of subalgebras is the class of solvable Lie algebras [17] Theorem 2.7. The following result shows that, given a flag of subalgebras, it is always possible to construct a fuzzy Lie subalgebra whose associated family of t-cut sets recovers exactly the original flag.

Proposition 5. 

Let $\mathfrak{g}$ be a Lie algebra that has a flag of subalgebras

$$0={\mathfrak{g}}_n<{\mathfrak{g}}_{n-1}<\dots <{\mathfrak{g}}_0=\mathfrak{g}.$$

Then there exists a fuzzy subalgebras $\mu :\mathfrak{g}\to [0,1]$ of $\mathfrak{g}$ such that there exists $t_0,t_1,\dots ,t_n\in [0,1]$ such that $U(\mu ,t_i)={\mathfrak{g}}_i.$

Proof. 

Let $\mu :\mathfrak{g}\to [0,1]$ be the fuzzy subset defined as

$$\mu \left(x\right)=\left\{\begin{array}{cc}{\displaystyle \frac{i}{n}},\hfill & \mathrm{if}x\in {\mathfrak{g}}_i\setminus {\mathfrak{g}}_{i+1},\hfill \\ 1,\hfill & \mathrm{if}x\in {\mathfrak{g}}_n,\hfill \end{array}\right.$$

for every $x\in \mathfrak{g}$ and $i\in \{0,\dots ,n-1\}.$ For every $i\in \{1,\dots ,n-1\}$, let $t_i\in [0,1]$ be such that ${\displaystyle \frac{i-1}{n}}<t_i\le {\displaystyle \frac{i}{n}}.$ Then

$$\begin{array}{cc}\hfill U(\mu ,t_i)& =\{x\in \mathfrak{g}\mid \mu \left(x\right)\ge t_i\}\hfill \\ \hfill & =\{x\in \mathfrak{g}\mid \mu \left(x\right)\ge {\displaystyle \frac{i}{n}}\}\hfill \\ \hfill & ={\mathfrak{g}}_i.\hfill \end{array}$$

Moreover, if $t_0=0$, we have $U(\mu ,t_0)=\{x\in \mathfrak{g}\mid \mu \left(x\right)\ge 0\}=\mathfrak{g}.$ As a consequence, for every $t\in [0,1]$, $U(\mu ,t)$ is a subalgebra of $\mathfrak{g}$ and hence $\mu :\mathfrak{g}\to [0,1]$ is a fuzzy subalgebra of $\mathfrak{g}$. □

Remark 1. 

Let μ be a fuzzy subalgebra of a Lie algebra $\mathfrak{g}$ and let ${\mathfrak{g}}^{\prime }$ be a subalgebra of $\mathfrak{g}$. Then the fuzzy subset ${\mu }^{\prime }:{\mathfrak{g}}^{\prime }\to [0,1]$ given by the restriction of μ to ${\mathfrak{g}}^{\prime }$ is a fuzzy subalgebra of ${\mathfrak{g}}^{\prime }$. Indeed, for every $x,y\in {\mathfrak{g}}^{\prime }$,

$${\mu }^{\prime }(x+y)=\mu (x+y)\ge \mu \left(x\right)\wedge \mu \left(y\right)={\mu }^{\prime }\left(x\right)\wedge {\mu }^{\prime }\left(y\right),$$

$${\mu }^{\prime }\left(\alpha x\right)=\mu \left(\alpha x\right)=\mu \left(x\right)={\mu }^{\prime }\left(x\right)$$

and

$${\mu }^{\prime }\left([x,y]\right)=\mu \left([x,y]\right)\ge \mu \left(x\right)\wedge \mu \left(y\right)={\mu }^{\prime }\left(x\right)\wedge {\mu }^{\prime }\left(y\right).$$

Moreover, the inclusion $\iota :{\mathfrak{g}}^{\prime }\to \mathfrak{g}$ of ${\mathfrak{g}}^{\prime }$ in $\mathfrak{g}$ is such that $\mu \left(x\right)\le {\mu }^{\prime }\left(\iota \left(x\right)\right).$

Consequently, given a Lie algebra $\mathfrak{g}$ that has a flag of subalgebras $0={\mathfrak{g}}_n<{\mathfrak{g}}_{n-1}<\dots <{\mathfrak{g}}_0=\mathfrak{g}$, there exists, for every $0\le i\le n$, a fuzzy subalgebra ${\mu }_i:{\mathfrak{g}}_i\to [0,1]$ of ${\mathfrak{g}}_i$. Moreover, the inclusion $\iota :{\mathfrak{g}}_i\to {\mathfrak{g}}_{i-1}$ is such that ${\mu }_i\left(x\right)\le {\mu }_{i-1}\left(\iota \left(x\right)\right)$, for every $x\in {\mathfrak{g}}_i.$

Then, for every $i\in \{0,\dots ,n\}$, the map $\iota :({\mathfrak{g}}_i,{\mu }_i)\to ({\mathfrak{g}}_{i-1},{\mu }_{i-1})$ given by the inclusion $\iota :{\mathfrak{g}}_i\to {\mathfrak{g}}_{i-1}$ of ${\mathfrak{g}}_i$ in ${\mathfrak{g}}_{i-1}$ is a morphism in ${\mathsf{Lie}}^F$.

Proposition 6. 

If a Lie algebra $\mathfrak{g}$ has a flag of subalgebras, then every epimorphic image of $\mathfrak{g}$ also has a flag of subalgebras.

In particular, if $\mathfrak{g}$ is a Lie algebra with flag of subalgebras

$$0={\mathfrak{g}}_n<{\mathfrak{g}}_{n-1}<\dots <{\mathfrak{g}}_0=\mathfrak{g}$$

and $f:\mathfrak{g}\to \mathfrak{h}$ is a surjective homomorphism of Lie algebras, then the chain

$$0\subseteq {\displaystyle \frac{{\mathfrak{g}}_{n-1}+K}{K}}\subseteq {\displaystyle \frac{{\mathfrak{g}}_{n-2}+K}{K}}\subseteq \dots \subseteq {\displaystyle \frac{\mathfrak{g}+K}{K}}\cong \mathfrak{h}$$

(5)

is a flag of subalgebras of $\mathfrak{h}$, where $K=ker\left(f\right)$. Now, let $\mu :\mathfrak{g}\to [0,1]$ the fuzzy subalgebra associated with $\mathfrak{g}$ by Proposition 5 and let $\nu :{\displaystyle \frac{\mathfrak{g}}{K}}\to [0,1]$ be the fuzzy subset of $\mathfrak{h}$ defined as

$$\nu (x+K)=\mu \left(x\right)$$

for every $x\in \mathfrak{g}.$ Then, $\nu$ is the fuzzy subalgebra associated with the flag (5) by Proposition 5. Indeed, let $x+K\in {\mathfrak{h}}_i\setminus {\mathfrak{h}}_{i+1}={\displaystyle \frac{{\mathfrak{g}}_i+K}{K}}\setminus {\displaystyle \frac{{\mathfrak{g}}_{i+1}+K}{K}}.$ Then

$$\nu (x+K)=\mu \left(x\right)={\displaystyle \frac{i}{n}}.$$

4. Nilpotent Fuzzy Lie Subalgebras

In this section, we address the following question: can nilpotency be formulated in the context of fuzzy algebra? We focus on nilpotency as a first step toward introducing a suitable measure of the degree to which a given property is satisfied by a class of fuzzy algebraic structures (for instance, nilpotency in the case of fuzzy Lie subalgebras). We provide two answers to this question. The first is more classical: we introduce a central series associated with a fuzzy subalgebra of a Lie algebra and define nilpotency by requiring that this series eventually stabilizes. This approach is inspired by the work of [9] and [18], where nilpotent subgroups were extended to the fuzzy setting; we propose here a similar generalisation for Lie algebras by introducing the notion of nilpotent fuzzy Lie subalgebras. The second approach takes into account the intrinsic vagueness of the fuzzy framework and proposes a way to measure the degree of nilpotency of a fuzzy Lie subalgebra. We note that a substantial part of this section is based on [8], which is here extended and complemented by the study of isomorphisms introduced in Section 3.

For any $x,y_1,y_2,\dots ,y_k\in \mathfrak{g}$, we denote by

$$[y_1,y_2,\dots ,y_k,x]:=[y_1,[y_2,[\dots ,[y_k,x]]\dots ]]$$

the iterated Lie bracket. Recall that a Lie algebra $\mathfrak{g}$ is said to be nilpotent if there exists $k\in \mathbb{N}$ such that

$$[y_1,y_2,\dots ,y_k,x]=0\mathrm{for}\mathrm{all}x,y_1,\dots ,y_k\in \mathfrak{g}.$$

(6)

This condition is equivalent to the vanishing of the lower central series of $\mathfrak{g}$, and hence it characterizes the nilpotency of $\mathfrak{g}$. Let $\mu \le \mathfrak{g}$ be a fuzzy Lie subalgebra. We define the lower central series associated to $\mu$ inductively as follows: we put

$$\begin{array}{cc}\hfill Z_0\left(\mu \right)& =\left\{0_{\mathfrak{g}}\right\},\hfill \\ \hfill Z_1\left(\mu \right)& =\{x\in \mathfrak{g}\mid \mu \left([y,x]\right)=\mu \left(0_{\mathfrak{g}}\right)\mathrm{for}\mathrm{any}y\in \mathfrak{g}\}\hfill \end{array}$$

and, inductively, for every $k\ge 2$,

$$Z_k\left(\mu \right)=\{x\in \mathfrak{g}\mid \mu \left([y_1,y_2,\dots ,y_k,x]\right)=\mu \left(0_{\mathfrak{g}}\right),\mathrm{for}\mathrm{any}{y}_1,\dots ,y_k\in \mathfrak{g}\}.$$

It is straightforward to verify that the sequence ${\left(Z_k\left(\mu \right)\right)}_{k\ge 0}$ is increasing, i.e. $Z_k\left(\mu \right)\subseteq Z_{k+1}\left(\mu \right)$, for all $k\ge 0$.

Definition 7. 

A fuzzy Lie subalgebra μ of a Lie algebra $\mathfrak{g}$ over a field $\mathbb{F}$ is called nilpotent fuzzy Lie subalgebra of $\mathfrak{g}$ if there exists $k\ge 0$ such that $Z_k\left(\mu \right)=\mathfrak{g}.$ The smallest k such that $Z_k\left(\mu \right)=\mathfrak{g}$ is called fuzzy nil-index of μ.

As one would expect, for a nilpotent Lie algebra, fuzzy nilpotency is trivially necessary, as the following result demonstrates.

Proposition 7. 

If μ is a fuzzy Lie subalgebra of a nilpotent Lie algebra $\mathfrak{g}$, then μ is a fuzzy nilpotent Lie subalgebra. Moreover, they have the same nil-index.

Proof. 

Since $\mathfrak{g}$ is nilpotent, then there exists an integer $k\ge 1$ such that ${\mathfrak{g}}^k=\left\{0_{\mathfrak{g}}\right\}$, with ${\mathfrak{g}}^{k-1}\ne \left\{0_{\mathfrak{g}}\right\}$. Hence, for every $y_1,\dots ,y_k\in \mathfrak{g}$, $[y_1,\dots ,y_k]=0_{\mathfrak{g}}$. Then, for every $x\in \mathfrak{g}$, $[y_1,\dots ,y_{k-1},x]=0_{\mathfrak{g}}$. This implies that $\mu \left([y_1,\dots ,y_{k-1},x]\right)=\mu \left(0_{\mathfrak{g}}\right)$, and statement is proved beacause $Z_k\left(\mu \right)=\mathfrak{g}$. □

We show that the converse is not true with the following example.

Example 2. 

Let $\mathfrak{g}$ be the unique 2-dimensional non-abelian Lie algebra over the filed $\mathbb{R}$, with basis the set $\{e_1,e_2\}$, and brackets given by $[e_1,e_2]=e_1$. It is known in the literature that $\mathfrak{g}$ is not nilpotent, but with this example we want to show that it is possible to define a fuzzy nilpotent fuzzy subalgebra μ on a non-nilpotent Lie algebra $\mathfrak{g}$. Let us define a fuzzy subalgebra $\mu \le \mathfrak{g}$ with the following membership function:

$$\mu \left(x\right)=\left\{\begin{array}{c}0.5\mathit{if}x=0\vee x=\alpha e_1,\alpha \in \mathbb{R},\hfill \\ 0.1\mathit{otherwise}.\hfill \end{array}\right.$$

We want to check whether μ is fuzzy nilpotent and, if it is the case, to compute the fuzzy nil-index of μ. In order to do this, we need to find the ascending central series associated with μ, and see if and when $Z_k\left(\mu \right)=\mathfrak{g}$. Of course, by deifnition, $Z_0\left(\mu \right)=\left\{0_{\mathfrak{g}}\right\}$. The first non-trivial set of the series is the subset

$$Z_1\left(\mu \right)=\{x\in \mathfrak{g}\mid \mu \left([y,x]\right)=\mu \left(0_{\mathfrak{g}}\right)\mathit{for}\mathit{any}y\in \mathfrak{g}\}.$$

Let $x=x_1{e}_1+x_2{e}_2$ and $y=y_1{e}_1+y_2{e}_2$ two generic elements of $\mathfrak{g}$. Let us compute $[y,x]$. We have that

$$\begin{array}{cc}\hfill [y,x]& =[y_1{e}_1+y_2{e}_2,x_1{e}_1+x_2{e}_2]\hfill \\ \hfill & =y_1{x}_2[e_1,e_2]+y_2{x}_2[e_2,e_1]\hfill \\ \hfill & =y_1{x}_2[e_1,e_2]-y_2{x}_2[e_1,e_2]\hfill \\ \hfill & =(y_1{x}_2-y_2{x}_2)[e_1,e_2]\hfill \\ \hfill & =(y_1{x}_2-y_2{x}_2)e_1.\hfill \end{array}$$

Hence, for every $x,y\in \mathfrak{g}$, we have that $\mu \left([y,x]\right)=\mu \left(\alpha e_1\right)=\mu \left(0_{\mathfrak{g}}\right)$. This means that $Z_1\left(\mu \right)=\mathfrak{g}$, thus μ is fuzzy nilpotent with fuzzy nil-index equal to 1.

We now want to introduce a sort of fuzzy measure for the nilpotency of a fuzzy Lie algebra. In particular, we are looking for a function $N:\mathcal{V}(\mu ,\mathfrak{g})\to [0,1]$, where $\mathcal{V}(\mu ,\mathfrak{g})=\{\mu :\mathfrak{g}\to [0,1]\mid \mu \text{non-constant}\mathrm{fuzzy}\mathrm{Lie}\mathrm{subalgebra}\mathrm{of}\mathfrak{g}\}$, such that $N\left(\mu \right)=1$ when $\mu$ is a nilpotent fuzzy Lie subalgebra of $\mathfrak{g}$, and $N\left(\mu \right)\in [0,1]$ otherwise.

To this end, we set ${\mathfrak{g}}^0=\mathfrak{g}$ and ${\mathfrak{g}}^1=[\mathfrak{g},\mathfrak{g}]$. By definition, the following inequality holds for every $x,y\in \mathfrak{g}$

$$\mu \left(\right[x,y\left]\right)\ge \mu \left(x\right)\wedge \mu \left(y\right).$$

In the next term of the series, ${\mathfrak{g}}^2:=[\mathfrak{g},{\mathfrak{g}}^1]$ this inequality extends as follows:

$$\begin{array}{cc}\hfill \mu \left(\right[x,[y,z]\left]\right)& \ge \mu \left(x\right)\wedge \mu \left(\right[y,z\left]\right)\hfill \\ \hfill & =\mu \left(x\right)\wedge \left(\mu \right(y)\wedge \mu (z\left)\right)\hfill \\ \hfill & =\mu \left(x\right)\wedge \mu \left(y\right)\wedge \mu \left(z\right).\hfill \end{array}$$

More generally, for each for $k\ge 3$, we define recursively ${\mathfrak{g}}^k:=[\mathfrak{g},{\mathfrak{g}}^{k-1}]$. Then, for any $x_1,...,x_k\in \mathfrak{g},$ we have

$$\mu \left([x_1,...,x_{k-1},x_k]\right)\ge \underset{i=1}{\overset{k}{\bigwedge }}\mu \left(x_i\right).$$

To emphasize this, we consider the minimum of the possible membership degrees for a k-tuple of elements, that is ${\bigwedge }_{x_i\in \mathfrak{g}}\mu \left([x_1,x_2,...,x_k]\right)$. If $\mathfrak{g}$ is nilpotent, then there exists $k\ge 1$, called nil-index of $\mathfrak{g}$ such that ${\mathfrak{g}}^{k-1}\ne \left\{0_{\mathfrak{g}}\right\}$ and ${\mathfrak{g}}^k=\left\{0_{\mathfrak{g}}\right\}$. We observe that if $\mathfrak{g}$ is nilpotent with nil-index k, then every fuzzy Lie subalgebra $\mu$ of $\mathfrak{g}$ is nilpotent with nil-index k. To quantify the extent to which a fuzzy Lie subalgebra approaches nilpotency, we define a fuzzy measure of nilpotency. This measure reflects how the membership values of higher-order commutators behave in the fuzzy setting. Formally, we put

$$N\left(\mu \right)=\underset{k\to \infty }{lim}{\displaystyle \frac{{\bigwedge }_{x_i\in \mathfrak{g}}\mu \left([x_1,x_2,...,x_k]\right)}{\mu \left(0_{\mathfrak{g}}\right)}}.$$

(7)

It is clear that, since $\mu \left(x\right)\le \mu \left(0_{\mathfrak{g}}\right)$ for every $x\in \mathfrak{g}$, then $N\left(\mu \right)\in [0,1]$ and, in particular, if $\mathfrak{g}$ is nilpotent then $N\left(\mu \right)=1$ for every fuzzy Lie subalgebra $\mu$ of $\mathfrak{g}$. In this formula, the numerator takes the minimum membership degree over all iterated Lie brackets of length k for two reasons. First, since $\mu \left(0_{\mathfrak{g}}\right)\ge \mu \left(x\right)$ for all $x\in \mathfrak{g}$, taking the maximum would trivially yield $N\left(\mu \right)=1$, which would defeat the purpose. Second, we want to capture the worst-case behaviour of iterated brackets, that is, how far they can be from the zero vector in terms of membership degree. This is then normalised by $\mu \left(0_{\mathfrak{g}}\right)$, the membership degree of the zero vector, which serves as a natural reference value. The only reason for considering the limit in Equation (7) as $k\to \infty$ is to theoretically extend this measure to infinite-dimensional Lie algebras, and moreover because the lower central series does not necessarily terminate a priori.

Remark 2. 

We briefly explain why, at least in the present manuscript, we have not introduced an analogue of the measure N for fuzzy subgroups. In the Lie algebra setting, nilpotency admits two equivalent characterisations: via the lower central series terminating at the trivial subalgebra, and via the condition expressed in Equation (6). The same equivalence does not hold in the group-theoretic setting. Indeed, for groups, the condition in Equation (6), where the bracket $[x,y]$ denotes the commutator $xy{x}^{-1}{y}^{-1}$, characterises the class of k-Engel groups, which coincides with the class of nilpotent groups only in the finite case. As soon as one moves to finitely generated groups, this equivalence breaks down. Since our measure N is built precisely on the condition in Equation (6), extending it to the group setting would capture Engel-type behaviour rather than nilpotency proper, making the analogy less meaningful. For a thorough treatment of nilpotent groups and Engel groups, we refer the reader to [19,20,21].

Conversely, even if $\mathfrak{g}$ is not nilpotent, a fuzzy Lie subalgebra $\mu$ may still satisfy $N\left(\mu \right)=1.$ In particular, we require that every $\mu \in \mathcal{V}(\mu ,\mathfrak{g})$ has to be not constant since if $\mu \left(x\right)=m\in ]0,1[$ for every $x\in \mathfrak{g}$, then $N\left(\mu \right)=1$. Moreover, if $\mu$ is a nilpotent fuzzy Lie subalgebra of $\mathfrak{g}$, then $N\left(\mu \right)=1.$

Example 3. 

This example is the Example 1.3 in [6]. Let $\mathfrak{g}={\mathbb{R}}^3$ the 3–dimensional real vector space with lie bracket $[x,y]=x\times y$, where $x,y\in {\mathbb{R}}^3$, that is the classical cross product. Then $\mathfrak{g}=({\mathbb{R}}^3,\times )$ is a real Lie algebra. We define a fuzzy set μ on $\mathfrak{g}$ by

$$\mu \left(x\right)=\left\{\begin{array}{cc}0.9\hfill & \mathit{if}x=(0,0,0),\hfill \\ 0.6\hfill & \mathit{if}x=(a,0,0),a\ne 0,\hfill \\ 0.2\hfill & \mathit{otherwise}.\hfill \end{array}\right.$$

One can easily check that μ is a fuzzy Lie algebra. With the values above, we have $\mu \left(e_1\right)=0.6$ and $\mu \left(e_2\right)=\mu \left(e_3\right)=0.2$, then ${min}_{e_i\in \mathcal{B}}=0.2=min\mu \left(x\right)$, for every $x\in {\mathbb{R}}^3\setminus \left\{0_{{\mathbb{R}}^3}\right\}$. Moreover, using Theorem 2, it is straightforward to see that $\mathcal{B}=\left\{e_1,e_2,e_3\right\}$ is a fuzzy basis of μ.

Now, $\mathfrak{g}$ is not a nilpotent Lie algebra. Indeed, the non-zero brackets are

$$[e_1,e_2]=e_3,[e_1,e_3]=-e_2,\mathit{and}[e_2,e_3]=e_1.$$

Hence, the k-th term of lower central series is $\mathfrak{g}$, for every $k\ge 1$. Then ${\bigwedge }_{x_i\in \mathfrak{g}}\mu \left([x_1,\dots ,x_k]\right)=0.2$, for every $k\ge 1$, and this implies that $N\left(\mu \right)={\displaystyle \frac{0.2}{0.9}}={\displaystyle \frac{2}{9}}$.

The fuzzy nilpotent measure N induces an order relation on the set of fuzzy Lie subalgebras of a fixed fuzzy Lie algebra $\mathfrak{g}$. Specifically, for any ${\mu }_1,{\mu }_2\le \mathfrak{g}$ we define

$${\mu }_1{\le }_N{\mu }_2\mathrm{if},\mathrm{and}\mathrm{only}\mathrm{if},N\left({\mu }_1\right)\le N\left({\mu }_2\right).$$

Then, if ${\mu }_1{\le }_N{\mu }_2$ holds, we say that ${\mu }_1$ is less nilpotent than ${\mu }_2$. More precisely, we have the following.

Theorem 3. 

${\le }_N$ is a total order on $\mathcal{V}(\mu ,\mathfrak{g})$.

In particular, since ${\le }_N$ is a total order, it induces a lattice structure on $\mathcal{V}(\mu ,\mathfrak{g})$.

4.1. Properties of N

Let $\mu$ and $\nu$ be two fuzzy subalgebras of a Lie algebra $\mathfrak{g}$. We define

$$\begin{array}{cc}\hfill \mu \wedge \nu & :\mathfrak{g}\to [0,1]\hfill \\ \hfill x& \mapsto \mu \left(x\right)\wedge \nu \left(x\right)\hfill \end{array}$$

and

$$\begin{array}{cc}\hfill \mu +\nu & :\mathfrak{g}\to [0,1]\hfill \\ \hfill x& \mapsto \mu \left(x\right)+\nu \left(x\right)-\mu \left(x\right)·\nu \left(x\right).\hfill \end{array}$$

Proposition 8 

([6] Theorem 1.2). The fuzzy sets $\mu \wedge \nu$ and $\mu +\nu$ are fuzzy ideals of $\mathfrak{g}$.

Proposition 9. 

Let $\mu ,\nu$ be fuzzy ideals of $\mathfrak{g}$, with μ less nilpotent than ν (i.e. $N\left(\mu \right)\le N\left(\nu \right))$. Then

$$N(\mu \cap \nu )\left\{\begin{array}{cc}=N\left(\mu \right)\hfill & \mathit{if}\mu \left(0_{\mathfrak{g}}\right)=\nu \left(0_{\mathfrak{g}}\right)\hfill \\ \le N\left({arg}_{\mu ,\nu }min\left\{\mu \left(0_{\mathfrak{g}}\right),\nu \left(0_{\mathfrak{g}}\right)\right\}\right)\hfill & \mathit{otherwise}.\hfill \end{array}\right..$$

Proof. 

We have

$$\begin{array}{cc}\hfill N(\mu \wedge \nu )& =\underset{k\to \infty }{lim}\underset{x_i\in \mathfrak{g}}{\bigwedge }{\displaystyle \frac{(\mu \wedge \nu )\left([x_1,...,x_k]\right)}{(\mu \wedge \nu )\left(0_{\mathfrak{g}}\right)}}\hfill \\ \hfill & =\underset{k\to \infty }{lim}\underset{x_i\in \mathfrak{g}}{\bigwedge }{\displaystyle \frac{\mu \left([x_1,...,x_k]\right)\wedge \nu \left([x_1,...,x_k]\right)}{\mu \left(0_{\mathfrak{g}}\right)\wedge \nu \left(0_{\mathfrak{g}}\right)}}.\hfill \end{array}$$

If $\mu \left(0_{\mathfrak{g}}\right)=\nu \left(0_{\mathfrak{g}}\right)$, then

$$\underset{x_i\in \mathfrak{g}}{\bigwedge }\mu ([x_1,...,x_k)\le \underset{x_i\in \mathfrak{g}}{\bigwedge }\nu ([x_1,...,x_k]$$

since $N\left(\mu \right)\le N\left(\nu \right).$ As a consequence,

$$\begin{array}{cc}\hfill N(\mu \wedge \nu )& =\underset{k\to \infty }{lim}\underset{x_i\in \mathfrak{g}}{\bigwedge }{\displaystyle \frac{\mu \left([x_1,...,x_k]\right)\wedge \nu \left([x_1,...,x_k]\right)}{\mu \left(0_{\mathfrak{g}}\right)\wedge \nu \left(0_{\mathfrak{g}}\right)}}\hfill \\ \hfill & =\underset{k\to \infty }{lim}\underset{x_i\in \mathfrak{g}}{\bigwedge }{\displaystyle \frac{\mu \left([x_1,...,x_k]\right)}{\mu \left(0_{\mathfrak{g}}\right)}}\hfill \\ \hfill & =N\left(\mu \right).\hfill \end{array}$$

If $\mu \left(0_{\mathfrak{g}}\right)\le \nu \left(0_{\mathfrak{g}}\right)$, then

$$\begin{array}{cc}\hfill N(\mu \wedge \nu )& =\underset{k\to \infty }{lim}\underset{x_i\in \mathfrak{g}}{\bigwedge }{\displaystyle \frac{\mu \left([x_1,...,x_k]\right)\wedge \nu \left([x_1,...,x_k]\right)}{\mu \left(0_{\mathfrak{g}}\right)\wedge \nu \left(0_{\mathfrak{g}}\right)}}\hfill \\ \hfill & =\underset{k\to \infty }{lim}\underset{x_i\in \mathfrak{g}}{\bigwedge }{\displaystyle \frac{\mu \left([x_1,...,x_k]\right)\wedge \nu \left([x_1,...,x_k]\right)}{\mu \left(0_{\mathfrak{g}}\right)}}\hfill \\ \hfill & \le \underset{k\to \infty }{lim}\underset{x_i\in \mathfrak{g}}{\bigwedge }{\displaystyle \frac{\mu \left([x_1,...,x_k]\right)}{\mu \left(0_{\mathfrak{g}}\right)}}\hfill \\ & =N\left(\mu \right).\hfill \end{array}$$

□

Proposition 10. 

Let $\mu ,\nu$ be fuzzy ideals of $\mathfrak{g}$. Then

$$N(\mu +\nu )\le N\left(\mu \right)+N\left(\nu \right)-\kappa (\mu ,\nu ),$$

where we set

$$\kappa (\mu ,\nu )=\underset{k\to \infty }{lim}\underset{x_i\in \mathfrak{g}}{\bigvee }{\displaystyle \frac{\mu \left([x_1,\dots ,x_k]\right)\nu \left([x_1,\dots ,x_k]\right)}{\mu \left(0_{\mathfrak{g}}\right)\nu \left(0_{\mathfrak{g}}\right)}}.$$

In particular, if μ is less nilpotent then ν, hence $N(\mu +\nu )\le 2N\left(\nu \right)-\kappa (\mu ,\nu )$.

Proof. 

We have

$$\begin{array}{cc}\hfill N(\mu +\nu )& =\underset{k\to \infty }{lim}\underset{x_i\in \mathfrak{g}}{\bigwedge }{\displaystyle \frac{(\mu +\nu )\left([x_1,...,x_k]\right)}{(\mu +\nu )\left(0_{\mathfrak{g}}\right)}}\hfill \\ \hfill & =\underset{k\to \infty }{lim}\underset{x_i\in \mathfrak{g}}{\bigwedge }{\displaystyle \frac{\mu \left([x_1,...,x_k]\right)+\nu \left([x_1,...,x_k]\right)-\mu \left([x_1,...,x_k]\right)·\nu \left([x_1,...,x_k]\right)}{\mu \left(0_{\mathfrak{g}}\right)+\nu \left(0_{\mathfrak{g}}\right)-\mu \left(0_{\mathfrak{g}}\right)·\nu \left(0_{\mathfrak{g}}\right)}}\hfill \\ \hfill & =\underset{k\to \infty }{lim}\underset{x_i\in \mathfrak{g}}{\bigwedge }{\displaystyle \frac{\mu \left([x_1,...,x_k]\right)}{\mu \left(0_{\mathfrak{g}}\right)+\nu \left(0_{\mathfrak{g}}\right)-\mu \left(0_{\mathfrak{g}}\right)·\nu \left(0_{\mathfrak{g}}\right)}}+\underset{k\to \infty }{lim}\underset{x_i\in \mathfrak{g}}{\bigwedge }{\displaystyle \frac{\nu \left([x_1,...,x_k]\right)}{\mu \left(0_{\mathfrak{g}}\right)+\nu \left(0_{\mathfrak{g}}\right)-\mu \left(0_{\mathfrak{g}}\right)·\nu \left(0_{\mathfrak{g}}\right)}}\hfill \\ & -\underset{k\to \infty }{lim}\underset{x_i\in \mathfrak{g}}{\bigwedge }{\displaystyle \frac{\mu \left([x_1,...,x_k]\right)·\nu \left([x_1,...,x_k]\right)}{\mu \left(0_{\mathfrak{g}}\right)+\nu \left(0_{\mathfrak{g}}\right)-\mu \left(0_{\mathfrak{g}}\right)·\nu \left(0_{\mathfrak{g}}\right)}}\hfill \\ \hfill & =\underset{k\to \infty }{lim}\underset{x_i\in \mathfrak{g}}{\bigwedge }{\displaystyle \frac{\mu \left([x_1,...,x_k]\right)}{\mu \left(0_{\mathfrak{g}}\right)+\nu \left(0_{\mathfrak{g}}\right)-\mu \left(0_{\mathfrak{g}}\right)·\nu \left(0_{\mathfrak{g}}\right)}}+\underset{k\to \infty }{lim}\underset{x_i\in \mathfrak{g}}{\bigwedge }{\displaystyle \frac{\nu \left([x_1,...,x_k]\right)}{\mu \left(0_{\mathfrak{g}}\right)+\nu \left(0_{\mathfrak{g}}\right)-\mu \left(0_{\mathfrak{g}}\right)·\nu \left(0_{\mathfrak{g}}\right)}}\hfill \\ & +\underset{k\to \infty }{lim}\underset{x_i\in \mathfrak{g}}{\bigvee }{\displaystyle \frac{\mu \left([x_1,...,x_k]\right)·\nu \left([x_1,...,x_k]\right)}{\mu \left(0_{\mathfrak{g}}\right)+\nu \left(0_{\mathfrak{g}}\right)-\mu \left(0_{\mathfrak{g}}\right)·\nu \left(0_{\mathfrak{g}}\right)}}\hfill \\ \hfill & \le N\left(\mu \right)+N\left(\nu \right)-\kappa (\mu ,\nu ).\hfill \end{array}$$

Moreover, if $\mu$ is less nilpotent that $\nu$, then $N\left(\mu \right)\le N\left(\nu \right)$ and hence

$$N(\mu +\nu )\le 2N\left(\nu \right)+\kappa (\mu ,\nu ).$$

□

Let $\mu$ be a fuzzy subalgebra of a Lie algebra $\mathfrak{g}$ and let $\nu$ be a fuzzy subalgebra of a Lie algebra $\mathfrak{h}$. Moreover, let $f:(\mathfrak{g},\mu )\to (\mathfrak{h},\nu )$ be an isomorphism in ${Lie}^F.$ Then $\mu \left(0_{\mathfrak{g}}\right)=\nu \left(0_{\mathfrak{h}}\right).$ Moreover, let $x\in \mathfrak{g}$ be such that $\mu \left(x\right)={\bigwedge }_{x_i\in \mathfrak{g}}\mu \left([x_1,\dots ,x_k]\right)$. Then

$$\mu \left(x\right)=\nu \left(f\left(x\right)\right)=\underset{y_i\in \mathfrak{h}}{\bigwedge }\nu \left([y_1,\dots ,y_k]\right).$$

Indeed, if $\nu \left(f\left(x\right)\right)>{\bigwedge }_{y_i\in \mathfrak{h}}\nu \left([y_1,\dots y_k]\right)$, then there exists $y\in \mathfrak{h}$ such that $\nu \left(y\right)<\nu \left(f\right(x\left)\right)$ and hence $\mu \left(f^{-1}\left(y\right)\right)=\nu \left(y\right)<\nu \left(f\left(x\right)\right)=\mu \left(x\right)$, which is a contradiction. By these arguments, one can deduce the following result.

Theorem 4. 

Let $(\mathfrak{g},\mu ),(\mathfrak{h},\nu )\in {\mathrm{Lie}}^F$ such that $\mathfrak{g}$ and $\mathfrak{h}$ finite dimensional. Let $f:(\mathfrak{g},\mu )\to (\mathfrak{h},\nu )$ an isomorphism in ${\mathrm{Lie}}^F$. Hence, $N\left(\mu \right)=N\left(\nu \right)$.