A019309(n)/4 for n >= 1.

Terms

a(0) =1a(1) =3a(2) =12a(3) =45a(4) =180a(5) =708a(6) =2832a(7) =11283a(8) =45132a(9) =180348a(10) =721392a(11) =2884860a(12) =11539440a(13) =46154928a(14) =184619712a(15) =738467565a(16) =2953870260a(17) =11815435908a(18) =47261743632a(19) =189046794180a(20) =756187176720