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// PG 591
n = 4
A = [4 1 0 0;1 4 1 0;0 1 4 1;0 0 1 4]
lam = spec(A)
// Since A is symmetric, all eigen values are real.
// The radii are all 1 or 2.
// The centers of all the circles are 4.
// All eigen values must all lie in the interval [2,6]
// Since the eigen values of inv(A) are the reciprocals of those of A,
// 1/6 <= mu <= 1/2
// Let inv(A) = B
B=inv(A);
norm(B,2)
n
i = 1:n;
j = 1:n;
// for j~i
// r = sum(abs(B(i,j)))
// norm(B,2) = r(B) <= o.5
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