Number of nodes of the backtrack tree for the n queens problem.

Terms

a(0) =1a(1) =2a(2) =3a(3) =6a(4) =17a(5) =54a(6) =153a(7) =552a(8) =2057a(9) =8394a(10) =35539a(11) =166926a(12) =856189a(13) =4674890a(14) =27358553a(15) =171129072a(16) =1141190303a(17) =8017021932a(18) =59365844491a(19) =461939618824