With reference to a question:Operations on power set, I wanted to generalize it for a power set of fuzzy sets. So, first let us try to define power set of fuzzy sets. Let $\mathscr{F}(X)$ be the set of all fuzzy sets $A: X \longrightarrow \left[ 0, 1 \right]$ defined over $X$.
Now, considering the operations of fuzzy union $u(a, b)$ and fuzzy intersection $i(a, b)$, where for certain element $x$ of $X$, the membership value of $x$ in the fuzzy sets $A$ and $B$ are $A(x) = a$ and $B(x) = b$ respectively, which must satisfy the following properties:-
Fuzzy Union:-
- The boundary conditions that $u(a, 0) = a$
- The monotonicity that if $b \leq d$ then $u(a, b) \leq u(b, d)$
- Commutativity that $u(a, b) = u(b, a)$
- Associativity that $u(a, u(b, d)) = u(u(a, b), d)$
Fuzzy Intersection:-
- The boundary conditions that $u(a, 1) = a$
- The monotonicity that if $b \leq d$ then $u(a, b) \leq u(b, d)$
- Commutativity that $u(a, b) = u(b, a)$
- Associativity that $u(a, u(b, d)) = u(u(a, b), d)$
So, considering these properties, the following things are clear:-
- The fuzzy power set $\mathscr{F}(X)$ of a crisp set $X$ is closed under both union and intersection.
- The operations of union and intersection are associative (by definition).
- The identities for both these operations exists and are (respectively for intersection and union) $X$ and $\phi_F$, where $X$ is a fuzzy set over $X$ defined as $\forall x \in X, X(x) = 1$ and $\phi_F$ is a fuzzy set over $X$ defined as $\forall x \in X, X(x) = 0$. This is a direct consequence of the boundary conditions in the definitions.
So, until now we can conclude that the fuzzy power set $\mathscr{F}(X)$ over a crisp set $X$ forms a monoid, infact a commutative monoid. And the arguments given in the Proof for crisp power set not being able to be a group can also be given here to prove that it does not form a group.
Now, the question is, after coming to this almost trivial result, in what all directions can I move further? This was a thought that came to my mind when I was travelling and once I was able to convince myself that this thought is absolutely correct, I am not able to think further. Any insights will be helpful for what to do next to extend this thought being in the fuzzy theory.