Settling the genus of the $n$-prism
In a 1977 paper, Ringel conjectured a formula for the genus of the $n$-prism $K_n\times K_2$ and verified its correctness for about five-sixths of all values $n$. We complete this calculation by showing that, with the exception of $n = 9$, Ringel's conjecture is true for $n \equiv 5, 9\pmod{12}$.