a(n) = Sum_{k=0..floor(n/2)} binomial(2*n-2*k,2*k)*2^(n-k).

Terms

a(0) =1a(1) =2a(2) =6a(3) =32a(4) =140a(5) =600a(6) =2632a(7) =11520a(8) =50320a(9) =219936a(10) =961376a(11) =4201984a(12) =18366144a(13) =80275840a(14) =350873728a(15) =1533616128a(16) =6703206656a(17) =29298713088a(18) =128060286464a(19) =559732334592