PREDICTION OF THE INDONESIA COMPOSITE INDEX (ICI) USING THE ARCH GARCH APPROACH AND THE FOURIER SERIES - Fadillah Mardianto, M. Fariz; Valida, Hanny; Putri, Farah Fauziah; Fauzi, Doni Muhammad; Pusporani, Elly (BAREKENG: Jurnal Ilmu Matematika dan Terapan, 2025)