a(n) = denominator((4^(2*n+1)*n!^2)/((2*n+3)*(2*n)!)).

Terms

a(0) =3a(1) =5a(2) =21a(3) =45a(4) =385a(5) =819a(6) =3465a(7) =7293a(8) =122265a(9) =255255a(10) =1062347a(11) =2204475a(12) =18253053a(13) =37702175a(14) =155451825a(15) =319929885a(16) =10518906825a(17) =21585857535a(18) =88482569175a(19) =181144476975