Consider the matrix
A=(1c⋯cc1⋱⋮⋮⋱⋱cc⋯c1)
for some c≥1. 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)|=|(N−1)c+11−c|=Ncc−1−1,
since the eigenvalues of A are 1−c and (N−1)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 1−c and (N−1)c+1 are the eigenvalues of A?
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