G.f.: A(x) = exp( Sum_{n>=1} 4^n * x^n/(n*(1+x^n)) ).

Terms

a(0) =1a(1) =4a(2) =12a(3) =52a(4) =204a(5) =804a(6) =3244a(7) =12948a(8) =51756a(9) =207108a(10) =828364a(11) =3313332a(12) =13253580a(13) =53014116a(14) =212055852a(15) =848224660a(16) =3392897772a(17) =13571588484a(18) =54286358988a(19) =217145432052a(20) =868581718860