Number of square lattice worms w_n.

Terms

a(0) =1a(1) =1a(2) =3a(3) =7a(4) =19a(5) =41a(6) =113a(7) =261a(8) =713a(9) =1681a(10) =4567a(11) =10993a(12) =29717a(13) =72493a(14) =195269a(15) =481261a(16) =1292729a(17) =3211263a(18) =8606801a(19) =21515135a(20) =57561815a(21) =144631085a(22) =386382359a(23) =974968645a(24) =2601469419a(25) =6587913395a(26) =17560287513a(27) =44605607915a(28) =118794020215a(29) =302552020141