Number of ternary strings of length n containing 00.

Terms

a(0) =0a(1) =0a(2) =1a(3) =5a(4) =21a(5) =79a(6) =281a(7) =963a(8) =3217a(9) =10547a(10) =34089a(11) =108955a(12) =345137a(13) =1085331a(14) =3392377a(15) =10549739a(16) =32667201a(17) =100782787a(18) =309946697a(19) =950599131a(20) =2908512145a(21) =8880484019a(22) =27064776729a(23) =82350874699a(24) =250212362465a(25) =759269653155