Number of square permutations of length n.

Terms

a(0) =1a(1) =2a(2) =6a(3) =24a(4) =104a(5) =464a(6) =2088a(7) =9392a(8) =42064a(9) =187296a(10) =828776a(11) =3644912a(12) =15937776a(13) =69317984a(14) =300009744a(15) =1292654304a(16) =5547021728a(17) =23715100480a(18) =101046014952a(19) =429209373296