Problem of the Week 1218
A Prime-Sum Circle
Arrange the numbers 1, 2, 3, ..., 20 in a circle so that the sum of any two neighboring numbers is prime.
Extra Credit. Resolve this with 20 replaced by n.
Source: Inspired by G. Hamilton, K.S. Kedlaya, and H. Picciotto, Square-sum pair partitions, College Math. Journal 46 (Sept. 2015) 264-269, but original with me in this prime version.