Number of 2-factors in P_4 X P_n.

Terms

a(0) =0a(1) =2a(2) =3a(3) =18a(4) =54a(5) =222a(6) =779a(7) =2953a(8) =10771a(9) =40043a(10) =147462a(11) =545603a(12) =2013994a(13) =7442927a(14) =27490263a(15) =101563680a(16) =375176968a(17) =1386004383a(18) =5120092320a(19) =18914660608a(20) =69873991466a(21) =258127586367a(22) =953569519203