Related to self-avoiding walks on square lattice.

Terms

a(0) =1a(1) =6a(2) =21a(3) =76a(4) =249a(5) =814a(6) =2521a(7) =7824a(8) =23473a(9) =70590a(10) =207345a(11) =610356a(12) =1765959a(13) =511006a(14) =14643993a(15) =41958852a(16) =118976633a(17) =337823486a(18) =951157365a(19) =2681163492a(20) =7505218171a(21) =21030311474