G.f.: 1/(1-2*x-6*x^2+4*x^3).

Terms

a(0) =1a(1) =2a(2) =10a(3) =28a(4) =108a(5) =344a(6) =1224a(7) =4080a(8) =14128a(9) =47840a(10) =164128a(11) =558784a(12) =1910976a(13) =6518144a(14) =22267008a(15) =75998976a(16) =259527424a(17) =885980672a(18) =3025129984a(19) =10328034304a(20) =35262925824a(21) =120393537536a(22) =411052492800