Twice the Euler or up/down numbers A000111.

Terms

a(0) =2a(1) =2a(2) =2a(3) =4a(4) =10a(5) =32a(6) =122a(7) =544a(8) =2770a(9) =15872a(10) =101042a(11) =707584a(12) =5405530a(13) =44736512a(14) =398721962a(15) =3807514624a(16) =38783024290a(17) =419730685952