Yulita, Henilia, and Michael Christian. “Pengaruh Indikator Daring: Surel Dan Layanan Pelanggan Terhadap Kesetiaan Pelanggan (Studi Empiris: Anggota Fitness First Di Jakarta)”. Jurnal Siasat Bisnis 19, no. 1 (April 9, 2016): 56–68. Accessed April 6, 2025. https://journal.uii.ac.id/JSB/article/view/4383.