void relax(double **u, double **rhs, int n){	int i,ipass,isw,j,jsw=1;	double h,h2;	h=1.0/(n-1);	h2=h*h;	for (ipass=1;ipass<=2;ipass++,jsw=3-jsw) {		isw=jsw;		for (j=2;j<n;j++,isw=3-isw)			for (i=isw+1;i<n;i+=2)				u[i][j]=0.25*(u[i+1][j]+u[i-1][j]+u[i][j+1]					+u[i][j-1]-h2*rhs[i][j]);	}}