Numbers of words on {0,1,2,3,4,} having no isolated zeros.

Terms

a(0) =1a(1) =4a(2) =17a(3) =73a(4) =313a(5) =1341a(6) =5745a(7) =24613a(8) =105449a(9) =451773a(10) =1935521a(11) =8292309a(12) =35526553a(13) =152205613a(14) =652091089a(15) =2793739205a(16) =11969154121a(17) =51279178141a(18) =219694231041a(19) =941231059125