## Counting Binary Matrices

Published on Saturday, 5th May 2018, 07:00 pm; Solved by 183;
Difficulty rating: 70%

### Problem 626

A binary matrix is a matrix consisting entirely of 0s and 1s. Consider the following transformations that can be performed on a binary matrix:

• Swap any two rows
• Swap any two columns
• Flip all elements in a single row (1s become 0s, 0s become 1s)
• Flip all elements in a single column

Two binary matrices $A$ and $B$ will be considered equivalent if there is a sequence of such transformations that when applied to $A$ yields $B$. For example, the following two matrices are equivalent:

$A=\begin{pmatrix} 1 & 0 & 1 \\ 0 & 0 & 1 \\ 0 & 0 & 0 \\ \end{pmatrix} \quad B=\begin{pmatrix} 0 & 0 & 0 \\ 1 & 0 & 0 \\ 0 & 0 & 1 \\ \end{pmatrix}$

via the sequence of two transformations "Flip all elements in column 3" followed by "Swap rows 1 and 2".

Define $c(n)$ to be the maximum number of $n\times n$ binary matrices that can be found such that no two are equivalent. For example, $c(3)=3$. You are also given that $c(5)=39$ and $c(8)=656108$.

Find $c(20)$, and give your answer modulo $1\,001\,001\,011$.