Number of 2n-step polygons on honeycomb.

Terms

a(0) =1a(1) =0a(2) =0a(3) =1a(4) =0a(5) =3a(6) =2a(7) =12a(8) =18a(9) =65a(10) =138a(11) =432a(12) =1074a(13) =3231a(14) =8718a(15) =25999a(16) =73650a(17) =220215a(18) =643546a(19) =1937877a(20) =5783700a(21) =17564727a(22) =53222094a(23) =163009086a(24) =499634508a(25) =1542392088a(26) =4770925446a(27) =14832934031a(28) =46227584010a(29) =144632622552