Trails of length n on square lattice.

Terms

a(0) =1a(1) =4a(2) =12a(3) =36a(4) =108a(5) =316a(6) =916a(7) =2628a(8) =7500a(9) =21268a(10) =60092a(11) =169092a(12) =474924a(13) =1329188a(14) =3715244a(15) =10359636a(16) =28856252a(17) =80220244a(18) =222847804a(19) =618083972a(20) =1713283628a(21) =4742946484a(22) =13123882524a(23) =36274940740a(24) =100226653420a(25) =276669062116a(26) =763482430316