a(n) = floor( n*(n - 1)/12 ).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =1a(6) =2a(7) =3a(8) =4a(9) =6a(10) =7a(11) =9a(12) =11a(13) =13a(14) =15a(15) =17a(16) =20a(17) =22a(18) =25a(19) =28a(20) =31a(21) =35a(22) =38a(23) =42a(24) =46a(25) =50a(26) =54a(27) =58a(28) =63a(29) =67