Wednesday, 4 June 2014

linear algebra - condition number of a matrix with diagonal ones and constant else.

Consider the matrix
A=(1ccc1ccc1)
for some c1. So A is a matrix with every entry equal to c, excepts for the diagonal entries, who equal 1.



If c=1, A is singular. Furthermore if c increases, the condition number of A decreases. I want to prove this, but I do not know how. Manually writing out the condition number seems a burden.



So my question is: how do I compute the condition number of A?







Edit:
The solution is
κ(A)=|λmax(A)λmin(A)|=|(N1)c+11c|=Ncc11,
since the eigenvalues of A are 1c and (N1)c+1, where N is the number of rows (or columns) of A.



However, I retrieved the eigenvalues by throwing a couple of matrices into WolframAlpha. So my question reduces to how to prove 1c and (N1)c+1 are the eigenvalues of A?

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