a(n) = n*(n-1)/2 + 2^(n-1) - 1.

Terms

a(0) =0a(1) =2a(2) =6a(3) =13a(4) =25a(5) =46a(6) =84a(7) =155a(8) =291a(9) =556a(10) =1078a(11) =2113a(12) =4173a(13) =8282a(14) =16488a(15) =32887a(16) =65671a(17) =131224a(18) =262314a(19) =524477a(20) =1048785a(21) =2097382a(22) =4194556a(23) =8388883a(24) =16777515a(25) =33554756a(26) =67109214a(27) =134218105a(28) =268435861a(29) =536871346