4-nonnesting permutations.

Terms

a(0) =1a(1) =2a(2) =6a(3) =24a(4) =120a(5) =720a(6) =5034a(7) =40087a(8) =356942a(9) =3500551a(10) =37343168a(11) =428886219a(12) =5257753614a(13) =68306562647a(14) =934747457369