a(n) = Sum_{d|n} phi(n/d)*C(2*d,d)/2.

Terms

a(0) =1a(1) =4a(2) =12a(3) =40a(4) =130a(5) =480a(6) =1722a(7) =6480a(8) =24336a(9) =92520a(10) =352726a(11) =1352640a(12) =5200312a(13) =20060040a(14) =77559060a(15) =300546720a(16) =1166803126a(17) =4537592928a(18) =17672631918a(19) =68923357200a(20) =269128940724