Trippy brain massages from Sam Palmer's artwork on the front of Issue 16

Issah Merchant discusses the geometric principles behind, and real-world applications of, curvature

Letter writing, hospital visits, and getting the family active are among the topics of discussion in this issue's Dear Dirichlet advice column

Infinitely many primes ending in 1, 3, 7 and 9 proved in typically Eulerian style.

Are you a winner?

Ever been stuck in motorway traffic which seems to have no reason? A model for traffic flow explains why.