Triangular matchstick numbers: a(n) = 3*n*(n+1)/2.

Terms

a(0) =0a(1) =3a(2) =9a(3) =18a(4) =30a(5) =45a(6) =63a(7) =84a(8) =108a(9) =135a(10) =165a(11) =198a(12) =234a(13) =273a(14) =315a(15) =360a(16) =408a(17) =459a(18) =513a(19) =570a(20) =630a(21) =693a(22) =759a(23) =828a(24) =900a(25) =975a(26) =1053a(27) =1134a(28) =1218a(29) =1305