Steffensen's bracket function [n,n-3].

Terms

a(0) =6a(1) =25a(2) =67a(3) =145a(4) =275a(5) =476a(6) =770a(7) =1182a(8) =1740a(9) =2475a(10) =3421a(11) =4615a(12) =6097a(13) =7910a(14) =10100a(15) =12716a(16) =15810a(17) =19437a(18) =23655a(19) =28525a(20) =34111a(21) =40480a(22) =47702a(23) =55850a(24) =65000a(25) =75231a(26) =86625a(27) =99267a(28) =113245a(29) =128650