a(n) = floor(binomial(n, 2)/2).

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =3a(5) =5a(6) =7a(7) =10a(8) =14a(9) =18a(10) =22a(11) =27a(12) =33a(13) =39a(14) =45a(15) =52a(16) =60a(17) =68a(18) =76a(19) =85a(20) =95a(21) =105a(22) =115a(23) =126a(24) =138a(25) =150a(26) =162a(27) =175a(28) =189a(29) =203