a(n) = Sum_{i=1..n} binomial(2*i,i).

Terms

a(0) =2a(1) =8a(2) =28a(3) =98a(4) =350a(5) =1274a(6) =4706a(7) =17576a(8) =66196a(9) =250952a(10) =956384a(11) =3660540a(12) =14061140a(13) =54177740a(14) =209295260a(15) =810375650a(16) =3143981870a(17) =12219117170a(18) =47564380970a(19) =185410909790a(20) =723668784230