Steffensen's bracket function [n,2].

Terms

a(0) =0a(1) =0a(2) =1a(3) =9a(4) =67a(5) =525a(6) =4651a(7) =47229a(8) =545707a(9) =7087005a(10) =102247051a(11) =1622631549a(12) =28091565547a(13) =526858344285