1. Robin’s criterion on divisibility
2. David W. Farmer, "Currently there are no reasons to doubt the Riemann Hypothesis," arXiv:2211.11671 [math.NT], 2022AD.
3. Guy Robin, “Grandes valeurs de la fonction somme des diviseurs et hypothése de Riemann.” J. Math. pures appl, 63(2): 187--213 (1984).
4. Abundant Numbers and the Riemann Hypothesis