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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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