Call a transformation f: A→A nonpermutational if f(B)={f(x): x∈B}=B implies |B| ≤ 1 for any B ⊆ A. (Equivalently, if A is finite, then f is nonpermutational if it has a unique fixed point Fix(f) and no more cycles saving the loop on Fix(f).)
The nonpermutational transformations of [n] do not form a semigroup. The question is, if T is a subsemigroup of Tn consisting of only nonpermutational transformations, how large can T be?
Partial results so far:
- For each n, there exists a semigroup of nonpermutational transformations having size ⌊e(n-1)!⌋. See http://arxiv.org/abs/1203.2873.
- We can show that the size of any such semigroup is at most n!. See http://arxiv.org/abs/1304.5714.
"soon" -- eh :S :)
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