Sum of spans of 2n-step polygons on square lattice.

Terms

a(0) =0a(1) =1a(2) =3a(3) =14a(4) =70a(5) =370a(6) =2028a(7) =11452a(8) =66172a(9) =389416a(10) =2326202a(11) =14070268a(12) =86010680a(13) =530576780a(14) =3298906810a(15) =20653559846a(16) =130099026600a(17) =823979294284