Number of mono-4-polyhexes with n cells.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =19a(5) =71a(6) =274a(7) =1117a(8) =4650a(9) =19819a(10) =85710a(11) =375712a(12) =1664203a(13) =7439593a(14) =33515758a(15) =152019560a(16) =693625265a(17) =3181528275a(18) =14661581030a(19) =67850297506a(20) =315187646601