a(1) = 1, a(m+1) = 2*Sum_{k=1..floor((m+1)/2)} a(m+1-k).

Terms

a(0) =1a(1) =2a(2) =4a(3) =12a(4) =32a(5) =96a(6) =280a(7) =840a(8) =2496a(9) =7488a(10) =22400a(11) =67200a(12) =201408a(13) =604224a(14) =1812112a(15) =5436336a(16) =16307328a(17) =48921984a(18) =146760960a(19) =440282880a(20) =1320833664a(21) =3962500992a(22) =11887458176a(23) =35662374528a(24) =106986989184a(25) =320960967552a(26) =962882499840