Ramanujan, Robin, Highly Composite Numbers, and the Riemann Hypothesis
  We provide an historical account of equivalent conditions for the Riemann Hypothesis arising from the work of Ramanujan and, later, Guy Robin on generalized highly composite numbers. The first part of the paper is on the mathematical background of our subject. The second part is on its history, which includes several surprises. 
Jean-Louis Nicolas and Jonathan Sondow
math.HO math.NT