SUBROUTINE resid(res,u,rhs,n) INTEGER n DOUBLE PRECISION res(n,n),rhs(n,n),u(n,n) INTEGER i,j DOUBLE PRECISION h,h2i h=1.d0/(n-1) h2i=1.d0/(h*h) do 12 j=2,n-1 do 11 i=2,n-1 res(i,j)=-h2i*(u(i+1,j)+u(i-1,j)+u(i,j+1)+u(i,j-1)-4.d0*u(i, *j))+rhs(i,j) 11 continue 12 continue do 13 i=1,n res(i,1)=0.d0 res(i,n)=0.d0 res(1,i)=0.d0 res(n,i)=0.d0 13 continue return END C (C) Copr. 1986-92 Numerical Recipes Software .