>> load hwk10
>> % is A symmetric?
>> norm(A-A')
??? Error using ==> norm
Use norm(full(S)) or norm(S,1) or norm(S,inf).

>> norm(A-A',inf)

ans =

     0

>> p = symmmd(A);
>> R = chol(A(p,p));
>> x = R\(R'\b(p));
>> x(p) = x;
>> norm(A*x-b)

ans =

   7.5578e-14

>> 
>> % type out pcg.m if you want to see an implementation of cg in MATLAB 
>> x = pcg(A,b,1e-4,1000);
pcg converged at iteration 36 to a solution with relative residual 7.4e-05
>> norm(A*x-b)

ans =

    0.0019

>> x = pcg(A,b,1e-8,1000);
pcg converged at iteration 68 to a solution with relative residual 6.8e-09
>> norm(A*x-b)

ans =

   1.7696e-07

>> 
>> % should use flops, cputime, but I forgot!!
>> 
>> diary off