a(n) = Sum_{k=0..n} C(2k+2,k).

Terms

a(0) =1a(1) =5a(2) =20a(3) =76a(4) =286a(5) =1078a(6) =4081a(7) =15521a(8) =59279a(9) =227239a(10) =873885a(11) =3370029a(12) =13027729a(13) =50469889a(14) =195892564a(15) =761615284a(16) =2965576714a(17) =11563073314a(18) =45141073924a(19) =176423482324a(20) =690215089744