Number of triangular lattice worms w_n.

Terms

a(0) =1a(1) =3a(2) =11a(3) =41a(4) =155a(5) =603a(6) =2361a(7) =9321a(8) =37015a(9) =147657a(10) =591227a(11) =2374539a(12) =9561487a(13) =38585555a(14) =156007667a(15) =631806555a(16) =2562434223a(17) =10405918209a(18) =42306525037a(19) =172180092143a(20) =701397054549