function laplacian(u, N, h)
U = reshape(u, N, N)
padded = circshift(U, (-1, 0)) .+ circshift(U, (1, 0)) .+
circshift(U, (0, -1)) .+ circshift(U, (0, 1)) .- 4 .* U
return vec(padded) ./ h^2
end