Floor( n(n-1)/7 ).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =2a(6) =4a(7) =6a(8) =8a(9) =10a(10) =12a(11) =15a(12) =18a(13) =22a(14) =26a(15) =30a(16) =34a(17) =38a(18) =43a(19) =48a(20) =54a(21) =60a(22) =66a(23) =72a(24) =78a(25) =85a(26) =92a(27) =100a(28) =108a(29) =116