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

Terms

a(0) =1a(1) =2a(2) =3a(3) =5a(4) =10a(5) =22a(6) =49a(7) =107a(8) =228a(9) =476a(10) =979a(11) =1993a(12) =4030a(13) =8114a(14) =16293a(15) =32663a(16) =65416a(17) =130936a(18) =261991a(19) =524117a(20) =1048386a(21) =2096942a(22) =4194073a(23) =8388355a(24) =16776940a(25) =33554132a(26) =67108539a(27) =134217377a(28) =268435078a(29) =536870506