# Wikipedia:Reference desk/Archives/Mathematics/2013 May 16

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# May 16

## Magic square type Combinatorics Problem

The problem is:

• In how many ways can you put 1 and -1 in the 16 places of a 4*4 square so that the product of any row or column is -1.

I figured out that -1 can be written by the product of 1s and -1s in only two ways:

-1 = 1 * 1 * 1 * -1 or -1 = -1 * -1 * -1 * 1

But I cannot figure out the solution. Any help appreciated. Solomon7968 (talk) 09:46, 16 May 2013 (UTC)

There are 2^16 = 65536 ways to distribute minuses in the the 4*4 square. The probability that any row or column multiplies to -1 is 50%. There are 4 rows and 4 columns. The probability that they all multiply to -1 is 2^(-8). So the answer to your question is probably (2^16)*(2^(-8))=2^8=256. Bo Jacoby (talk) 10:40, 16 May 2013 (UTC).
I am new to combinatorics. Will you explain how you got the 50% thing? Solomon7968 (talk) 10:43, 16 May 2013 (UTC)
Consider any row or column. There are 4 places. Each place can contain either +1 or -1. The sixteen possibilities are ++++ +++- ++-+ ++-- +-++ +-+- +--+ +--- -+++ -++- -+-+ -+-- --++ --+- ---+ ----. Eight out of these sixteen possibilities have an odd number of minuses and so multiplies to minus. (Namely +++- ++-+ +-++ +--- -+++ -+-- --+- ---+ ). So the probability that a random row multiplies to minus is 8/16=1/2=50%. Bo Jacoby (talk) 10:51, 16 May 2013 (UTC).
There's 512 ways. You can put any combination of +1 and -1 into the first 3x3 places. Then just place a +1 or -1 as appropriate into the last row or column to make the produce -1. The product of the three end columns will be te same as the three rows as they are determined by the product of all 9 numbers so there is no confusion about the last end number to put in. Dmcq (talk) 10:58, 16 May 2013 (UTC)
...and this is consistent with Bo's probabilistic approach once you take into account the fact that the 8 row/column products are not independent - once you know any 7 of them then the 8th is determined by the contraint that product of row products = product of column products. Correcting this in Bo's formula gives 2^16 / 2^7 = 2^9 = 512. Now for bonus points determine how many arrangements there are if reflections and rotations of an arrangement are not counted as distinct. Gandalf61 (talk) 13:07, 16 May 2013 (UTC)