a(n) = floor(n*(n-1)/9).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =2a(6) =3a(7) =4a(8) =6a(9) =8a(10) =10a(11) =12a(12) =14a(13) =17a(14) =20a(15) =23a(16) =26a(17) =30a(18) =34a(19) =38a(20) =42a(21) =46a(22) =51a(23) =56a(24) =61a(25) =66a(26) =72a(27) =78a(28) =84a(29) =90