a(n) = 4*A000984(n) - 3.

Terms

a(0) =1a(1) =5a(2) =21a(3) =77a(4) =277a(5) =1005a(6) =3693a(7) =13725a(8) =51477a(9) =194477a(10) =739021a(11) =2821725a(12) =10816621a(13) =41602397a(14) =160466397a(15) =620470077a(16) =2404321557a(17) =9334424877a(18) =36300541197a(19) =141381055197a(20) =551386115277