function ax = atv(b)

global m h uxc
h1 = h*h;

u = u1d_to_2d(m,b);
axt = zeros(m+1,m+1);

for i=2:m,
   for j=2:m
     axt(i,j) = (u(i-1,j) + u(i+1,j)+ u(i,j-1) + u(i,j+1) -4*u(i,j))/h1 ...
                + uxc * (u(i+1,j) - u(i-1,j))/(2*h);
   end
end 

ax = u2d_to_1d(m,axt);