a(n) = floor(n*(n-1)/13).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =1a(6) =2a(7) =3a(8) =4a(9) =5a(10) =6a(11) =8a(12) =10a(13) =12a(14) =14a(15) =16a(16) =18a(17) =20a(18) =23a(19) =26a(20) =29a(21) =32a(22) =35a(23) =38a(24) =42a(25) =46a(26) =50a(27) =54a(28) =58a(29) =62