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

Terms

a(0) =1a(1) =3a(2) =10a(3) =34a(4) =115a(5) =388a(6) =1309a(7) =4417a(8) =14905a(9) =50296a(10) =169720a(11) =572707a(12) =1932556a(13) =6521263a(14) =22005505a(15) =74255899a(16) =250570870a(17) =845532298a(18) =2853184279a(19) =9627852832a(20) =32488455385a(21) =109629815881a(22) =369937455865