Subanar . (1), Suryo Guritno (2), Zanzawi S. (3), Abdurakhman . (4) (1) Department of Mathematics, Gadjah Mada University, Jogjakarta, Indonesia., Indonesia,
(2) Department of Mathematics, Gadjah Mada University, Jogjakarta, Indonesia.,
(3) Department of Mathematics, Gadjah Mada University, Jogjakarta, Indonesia., Indonesia,
(4) Department of Mathematics, Gadjah Mada University, Jogjakarta, Indonesia., Indonesia