G.f. A(x) satisfies A(x) = 1 + x*A(x)^2*(A(x) + A(-x))/2.

Terms

a(0) =1a(1) =1a(2) =2a(3) =7a(4) =22a(5) =94a(6) =340a(7) =1579a(8) =6118a(9) =29746a(10) =120060a(11) =600934a(12) =2492028a(13) =12725756a(14) =53798888a(15) =278786739a(16) =1195684230a(17) =6265816042a(18) =27175425004a(19) =143671870034a(20) =628705751828