Equation Icon  x2 > x1 + 1 E2 y(x1, x2) = 2 y(x1,x2) - y(x1 - 1,x2)- y(x1 + 1,x2) + 2 y(x1,x2) - y(x1, x2- 1)- y(x1, x2 + 1) (55) x2 = x1 + 1 E2 y(x1, x2) = 2 y(x1,x2) - y(x1 - 1,x2)- y(x1,x2 - 1). (56)