Sum of sparse matrices - Matlab numerical error -

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i have following problem:

n=500;  = sparse(1:500,10*ones(n,1),0.005*ones(n,1),n,n);  a(:,300) = -0.005*ones(n,1);  >> sum(a,2)     ans = 0 sparse: 500-by-1  b = spdiags(0.45*ones(n,1),0,n,n);  b(:,34) = -0.45*ones(n,1);  b(34,34) = 0;  >> sum(b,2)     ans = 0 sparse: 500-by-1  c = a+b;  >>max(sum(c,2))       ans = (1,1) 4.3368e-18.

since sum of rows of , b 0, sum of rows of c should 0 too. large sparse matrices, not case. if e-18, need closer 0.

how can make matlab improve precision in kind of computations?

thank you


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