conjgrad

function x = conjgrad(A,b,x0,k)
    x = x0;
    d = b - A*x0;
    lastR = b - A*x0;
    for i=1:k
         if lastR == 0
             return
         end
         alfa=lastR' * lastR / (d' * A * d);
         x = x + alfa * d;
         r = lastR - alfa * A * d;
         beta = r' * r / (lastR' * lastR);
         d = r + beta*d;
         lastR = r;
    end
end