4
$\begingroup$

There are non-isomorphic finite groups with the same (complex) character table, as $D_4$ and $Q_8$.

$$\scriptsize\begin{array}{c|c} \text{class}&1&2A&2B&2C&4 \newline \text{size}&1&1&2&2&2 \newline \hline \rho_1 &1&1&1&1&1 \newline \rho_2 &1&1&-1&1&-1 \newline \rho_3 &1&1&1&-1&-1 \newline \rho_4 &1&1&-1&-1&1 \newline \rho_5 &2&-2&0&0&0 \newline \end{array} \ \ \ \ \ \ \ \ \ \ \ \ \ \ \begin{array}{c|c} \text{class}&1&2&4A&4B&4C \newline \text{size}&1&1&2&2&2 \newline \hline \rho_1 &1&1&1&1&1 \newline \rho_2 &1&1&-1&1&-1 \newline \rho_3 &1&1&1&-1&-1 \newline \rho_4 &1&1&-1&-1&1 \newline \rho_5 &2&-2&0&0&0 \newline \end{array}$$ But the character tables of $D_4$ and $Q_8$ are no more equal if we include the class types, as $(1,2A,2B,2C,4) \neq (1,2,4A,4B,4)$. A class is of type $nX$ if its elements has order $n$.

Question 1: Are there non-isomorphic groups with the same character table including class types?

Answer: Yes (see the comment of Derek Holt) and if in addition their conjugacy classes have the same power map, they are called Brauer pairs. Among the $2$-groups, the smallest order of a group in a Brauer pair is $2^8$, and among the $56092$ groups of order $2^8$, there are exactly ten Brauer pairs (see MR2680716 Theorem 2.6.2 page 136).

I am specifically interested in groups of square-free order.

Question 2: Is there a Brauer pair of square-free order groups?
(unless any two square-free order groups with same character table are isomorphic)

$\endgroup$
3
  • 2
    $\begingroup$ Yes, if you search for Brauer pairs you will find plenty of references. $\endgroup$
    – Derek Holt
    Mar 27, 2019 at 11:25
  • 1
    $\begingroup$ @DerekHolt: Thanks! The examples I found are all $p$-groups. I need to know if there are examples of square-free order. $\endgroup$ Mar 27, 2019 at 12:02
  • 1
    $\begingroup$ @MikkoKorhonen perhaps you should make your comment into an answer. $\endgroup$
    – Derek Holt
    Mar 30, 2019 at 5:36

1 Answer 1

7
$\begingroup$

(Turning my comments into an answer).

A finite group with all Sylow subgroups cyclic is called a $Z$-group.

According to review MR0470050 in MathSciNet, in [1] it is shown that a $Z$-group is determined by its character table. So the answer to question 2 would be no.

I do not have access to [1] and I have not seen it, if you are interested it seems you might have to look for a physical copy in a library somewhere.

However, perhaps with some effort you could figure out your own proof. We have a good understanding of the structure of $Z$-groups (in particular of groups of squarefree order). See for example [2] for an enumeration of all $Z$-groups of given order. I would guess the character tables of $Z$-groups are also known.

[1] Çelik, Özdem. On the character table of Z-groups. Mitt. Math. Sem. Giessen Heft 121 (1976), 75–77

[2] M.R. Murty and V.K. Murty. On groups of squarefree order. Math. Ann. 267 (1984), 299–309.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.