Found a solution in Stackexchage and quoting it here:

http://math.stackexchange.com/questions/435880/if-aat-is-the-zero-matrix-then-a-is-the-zero-matrix

AAt=(∑k=1naikbkj)=(∑k=1naikajk)

∑k=1naikaik=∑k=1na2ik

http://math.stackexchange.com/questions/435880/if-aat-is-the-zero-matrix-then-a-is-the-zero-matrix

if we put A=(aij)1≤i,j≤n , then At=(bij) , with bij=aji , so by definition:

If you now look at the main diagonal's general entry of the above, you get

So if AAt=0 then the above diagonal's entries are zero, but a sum of squared

*real*numbers is zero iff each number is zero, so...
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