a(n) = floor( n*(n-1)/5 ).

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =2a(5) =4a(6) =6a(7) =8a(8) =11a(9) =14a(10) =18a(11) =22a(12) =26a(13) =31a(14) =36a(15) =42a(16) =48a(17) =54a(18) =61a(19) =68a(20) =76a(21) =84a(22) =92a(23) =101a(24) =110a(25) =120a(26) =130a(27) =140a(28) =151a(29) =162