a(n) is the least perfect power > 2^n.

Terms

a(0) =4a(1) =4a(2) =8a(3) =9a(4) =25a(5) =36a(6) =81a(7) =144a(8) =289a(9) =529a(10) =1089a(11) =2116a(12) =4225a(13) =8281a(14) =16641a(15) =33124a(16) =66049a(17) =131769a(18) =263169a(19) =525625a(20) =1050625a(21) =2099601a(22) =4198401a(23) =8392609a(24) =16785409a(25) =33558849a(26) =67125249a(27) =134235396a(28) =268468225a(29) =536895241