def METHOD_NAME():
    grid = RasterModelGrid((3, 3))
    value_at_node = np.full(grid.number_of_nodes, 1.0)
    mat, rhs = get_core_node_matrix(grid, value_at_node)
    assert_array_equal(mat.toarray(), [[-4.0]])
    assert_array_equal(rhs, [[-4.0]])
    grid = RasterModelGrid((4, 4))
    value_at_node = np.full(grid.number_of_nodes, 1.0)
    mat, rhs = get_core_node_matrix(grid, value_at_node)
    assert_array_equal(
        mat.toarray(),
        [
            [-4.0, 1.0, 1.0, 0.0],
            [1.0, -4.0, 0.0, 1.0],
            [1.0, 0.0, -4.0, 1.0],
            [0.0, 1.0, 1.0, -4.0],
        ],
    )
    assert_array_equal(rhs, [[-2.0], [-2.0], [-2.0], [-2.0]])