Hammersley's polynomial p_n(1).

Terms

a(0) =1a(1) =1a(2) =2a(3) =7a(4) =41a(5) =376a(6) =5033a(7) =92821a(8) =2257166a(9) =69981919a(10) =2694447797a(11) =126128146156