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

Terms

a(0) =1a(1) =4a(2) =10a(3) =21a(4) =41a(5) =78a(6) =148a(7) =283a(8) =547a(9) =1068a(10) =2102a(11) =4161a(12) =8269a(13) =16474a(14) =32872a(15) =65655a(16) =131207a(17) =262296a(18) =524458a(19) =1048765a(20) =2097361a(21) =4194534a(22) =8388860a(23) =16777491a(24) =33554731a(25) =67109188a(26) =134218078a(27) =268435833a(28) =536871317a(29) =1073742258