Symmetric Binary Chess?

How many 8 × 8 8\times8 symmetric matrices containing exactly 9 9 ones and 55 55 zeroes exist?

Inspiration from misreading this question


The answer is 368648.

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1 solution

A symmetric matrix would have A T = A A^{T} = A . Note that in the transpose matrix, the diagonal elements remain where they are and other elements get 'mirrored' along the diagonal.If we divide the board into 3 parts, the diagonal, the lower half and the upper half, every arrangement of the lower half will have only one arrangement in the upper half such that A T = A A^{T}=A . For this reason, there cannot be an odd number of 1 s 1'^s outside the diagonal (lower + upper half). Each half has 28 28 blocks and the diagonal has 8 8 blocks. Therefore in total, there would be ( 28 4 ) ( 8 1 ) + ( 28 3 ) ( 8 3 ) + ( 28 2 ) ( 8 5 ) + ( 28 1 ) ( 8 7 ) = 368648 matrices \binom{28}{4}\binom{8}{1} + \binom{28}{3}\binom{8}{3} + \binom{28}{2}\binom{8}{5}+\binom{28}{1}\binom{8}{7} = 368648 \text{ matrices}

Can you make the solution a bit more clear please?

Ashley Shamidha - 5 years, 9 months ago

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Hi, certainly can. From which part did it become hard to comprehend? I'll explain from there.

Vishnu Bhagyanath - 5 years, 9 months ago

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