Exponential-convolution of natural numbers with themselves.

Terms

a(0) =1a(1) =4a(2) =14a(3) =44a(4) =128a(5) =352a(6) =928a(7) =2368a(8) =5888a(9) =14336a(10) =34304a(11) =80896a(12) =188416a(13) =434176a(14) =991232a(15) =2244608a(16) =5046272a(17) =11272192a(18) =25034752a(19) =55312384a(20) =121634816a(21) =266338304a(22) =580911104a(23) =1262485504a(24) =2734686208a(25) =5905580032a(26) =12717129728