# Probability of Words Summing to $1$ in $S_n$ or $PGL_2(n)$

Let $$G$$ be the symmetric group $$S_n$$ or the projective general linear group $$PGL_2(n)$$.

Let $$X$$ be a cyclically reduced word in the abstract variables $$x_1, x_2, \ldots,x_k$$, i.e. $$X$$ is a product containing $$x_1, x_2, \ldots,x_k$$ and their inverses, without any element appearing next to its own inverse in any cyclic permutation. (Only words with length $$4$$, $$6$$, $$8$$ are needed in my research.)

Consider the probability $$P$$ that the word sums to $$1$$, with each $$x_i$$ chosen uniformly and independently from $$G$$.

Question:

What are the upper bounds of $$\log_{|G|}P$$?

If $$\log_{|G|}P$$ converges when $$n→\infty$$, what's the value?

Answers are acceptable for either $$G=S_n$$ or $$G=PGL_2(n)$$.

Known:

If there's a variable occuring only once in $$X$$, then $$P$$ is exactly $$1/|G|$$.

If $$X=x_1^k$$, then the limit is $$-1/k$$ for symmetric groups by David E Speyer's argument.

As Richard Stanley pointed out, if $$X=x_1x_2x_1^{-1}x_2^{-1}$$, then $$P=|Conj(G)|/|G|$$. ($$|Conj(G)|$$ is the number of conjugacy classes of $$G$$)

The formula $$P=|Conj(G)|/|G|$$ holds for the words $$x_1x_1x_2x_2$$ and $$x_1x_2x_1x_2^{-1}$$ if all the characters of $$G$$ are real, and that's exactly the case for $$S_n$$ and $$PGL_2(n)$$.

• What is $\varepsilon$? It doesn't seem to appear anywhere else except in the clause "for each $\varepsilon >0$". – Noam D. Elkies Jun 8 '19 at 13:02
• I have edited the question: There should be no $ε$. – LeechLattice Jun 8 '19 at 13:03
• If $X = A^k$ then the limit is $-1/k$. The generating function for the probability that an element of $S_n$ obeys $g^k=1$ is $f_k(x) := \exp \left( \sum_{d|k} t^d/d \right)$. Taking the contour integral $\oint f_k(x) x^{-N-1} dx$ around a circle of radius $N^{1/k}$ gives asymptotics of the form $N^{-N/k} \exp(N/k+o(N))$. – David E Speyer Jun 8 '19 at 13:42
• A paper that is somewhat related is A. Nica, On the number of cycles of given length of a free word in several random permutations, Random Structures & Algorithms 5 (1994), 703-730. An unrelated comment: for $X=xyx^{-1}y^{-1}$, the limit is 1, since the number of commuting pairs in any finite group $G$ is $|G|\cdot k(G)$, where $k(G)$ is the number of conjugacy classes, and the number of conjugacy classes in $S_n$ is around $e^{c\sqrt{n}}$. – Richard Stanley Jun 9 '19 at 13:15
• As for $X=x_1x_1x_2x_2$, this is part of Exercise 7.69(h) of Enumerative Combinatorics, vol. 2. Possibly part (i) can solve the problem for $xy^kxy^{-k}$ and $xy^kx^{-1}y^k$, but I have not tried to do this. See also (f) for a possible approach to $X=x_1^{a_1}\cdots x_m^{a_m}$. – Richard Stanley Jun 9 '19 at 22:22