Steffensen's bracket function [n,3].

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =14a(5) =145a(6) =1450a(7) =15421a(8) =180894a(9) =2359225a(10) =34072850a(11) =540848341a(12) =9363767974a(13) =175619185105