The number of quaternary strings of length n containing 00.

Terms

a(0) =0a(1) =0a(2) =1a(3) =7a(4) =40a(5) =205a(6) =991a(7) =4612a(8) =20905a(9) =92935a(10) =407056a(11) =1762117a(12) =7556095a(13) =32148940a(14) =135892321a(15) =571232647a(16) =2389810360a(17) =9956870845a(18) =41335010911a(19) =171055514452a(20) =705891052825