Further simplifications to 2-D filter stability test

A further simplification to the stability test of the characteristic polynomial D(x(1),z(2)) of a two-dimensional (2-D) filter is achieved by the following two modifications in the test procedure, At first, the modified Jury table is used to construct the polynomial array and then the real form of Siljak's theorem applied. Secondly, the positivity test on Delta(n), the last entry of the polynomial array, is further reduced to tests on its factors D(x(1), 1), D(x(1), -1) and Delta(n-1)(-)(x(1)), which are of much lower orders in x(1).