function [a, r2] = sweep2(a,q)
[m,m1] = size(a);
j = q;
pv = 1./a(j,j);
g = a - a(:,j)*a(j,:)*pv;
g(j,:) = a(j,:)*pv;
g(:,j) = -a(:,j)*pv;
g(j,j) = pv;
a = g;
r2 = 1-a(m,m);