# Transformation terminology question

Given a transformation $t$ from the transformation semigroup $T_{n}$, if you take powers of $t$ under composition you get a length $s$ stem followed by a cycle. Permutations by definition have a length zero stem.

Are the terms "stem" and "cycle" common, or is some other terminology used in the literature?

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This question seems a bit ill-posed to me. There are two possible interpretations here.

(1) If $s$ is an element of a finite semigroup, then $s^i=s^{i+p}$ where there is a minimal such $i$ and $p$. What is the common terminology for $i,p$?

The "official" terminology is that $i$ is the index and $p$ is the period. Or one might call the elements $\lbrace s,..,s^{i-1}\rbrace$ the nilpotent part and $\lbrace s^i,..,s^{i+p-1}\rbrace$ the maximal subgroup or minimal ideal.

This in any event has nothing to do with transformations.

(2) You are interested in the analogue of cycle decomposition for a transformation. In this case, the situation is more complicated than a stem and cycle.

The transformation $f$ has a set of recurrent points (also called the eventual image) on which $f$ acts like a permutation and hence decomposes into cycles. There may be several of them. Attached to each cycle are some number of trees which are directed toward the cycle. There is a only a stem followed by a cycle if there is a point from which every other point can be reached. The terminology is not so standard for this setup. I think there is a good chapter in Peter Higgins book Techniques of semigroups on this sort of thing.

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s= [3, 3, 1, 0], s^2= [0, 0, 3, 3], s^3=[3, 3, 0, 0], s^4=[0, 0, 3, 3], ... The stem is [3, 3, 1, 0], the next two form the a cycle with period two. So "nilpotent" for [3, 3, 1, 0], and "maximal subgroup/minimal ideal" for {[0, 0, 3, 3], [3, 3, 0, 0]}? Hmm. I'll try to track down a copy of Higgins. –  Chad Brewbaker May 15 '13 at 19:08
Ben, you seem like just the person to ask a related question I posted on CSTheory: cstheory.stackexchange.com/questions/17326/name-this-digraph –  Chad Brewbaker May 15 '13 at 19:26
Yep, index and period seem to be standard: en.wikipedia.org/wiki/Monogenic_semigroup –  Chad Brewbaker May 15 '13 at 19:50
@Chad, it seems then that it is the cycle/stem in the semigroup you are interested in. The situation I refer to in 2 is s=[1,2,1,2]. Then on the level on {0,1,2,3} one has s cycles {1,2} and has two branches 0->1 and 2<-3. But on the semigroup it is still index 2, period 2. –  Benjamin Steinberg May 15 '13 at 19:58
I am not on cstheory stackexchange. The answer to your question is the directed power graph of the semigroup, or at least groupprops.subwiki.org/wiki/Directed_power_graph_of_a_group says this is the name in group theory for this. I think the undirected version is more commonly studied, see this paper sciencedirect.com/science/article/pii/S0012365X10000531 by Peter Cameron et al. –  Benjamin Steinberg May 15 '13 at 20:02