a(n) = denominator of binomial(2n,n)/4^n.

Terms

a(0) =1a(1) =2a(2) =8a(3) =16a(4) =128a(5) =256a(6) =1024a(7) =2048a(8) =32768a(9) =65536a(10) =262144a(11) =524288a(12) =4194304a(13) =8388608a(14) =33554432a(15) =67108864a(16) =2147483648a(17) =4294967296a(18) =17179869184a(19) =34359738368a(20) =274877906944a(21) =549755813888