13

This number is a prime.

"Remarkably, it turns out that for N = 13 the set of operators of 'exceptional' type (4) is empty, i.e., any Hamiltonian operator of order N = 13 can be taken by a contact transformation to an operator of type (1), (2) or (3)! We conjecture that the same holds for all N > 13." Reference: On Classification of Poisson Vertex Algebras by Alberto De Sole, Victor Kac, and Minoru Wakimoto. [Post]