a(n) = Sum_{d dividing n} binomial(2d,d).

Terms

a(0) =2a(1) =8a(2) =22a(3) =78a(4) =254a(5) =952a(6) =3434a(7) =12948a(8) =48642a(9) =185016a(10) =705434a(11) =2705178a(12) =10400602a(13) =40120040a(14) =155117794a(15) =601093338a(16) =2333606222a(17) =9075184872a(18) =35345263802a(19) =137846713906a(20) =538257877894