a(1) = 1; a(n+1) = Sum_{d|n} 4^(n/d - 1) * a(d).

Terms

a(0) =1a(1) =1a(2) =5a(3) =21a(4) =89a(5) =345a(6) =1405a(7) =5501a(8) =22033a(9) =87649a(10) =350405a(11) =1398981a(12) =5596345a(13) =22373561a(14) =89492141a(15) =357930301a(16) =1431711857a(17) =5726679153a(18) =22906712645a(19) =91626189381a(20) =366504720137