Number of ternary words of length n with exactly one 000.

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =4a(5) =16a(6) =60a(7) =212a(8) =728a(9) =2444a(10) =8064a(11) =26256a(12) =84576a(13) =270048a(14) =855936a(15) =2696080a(16) =8446912a(17) =26341696a(18) =81812544a(19) =253181888a(20) =781005440a(21) =2402311616a(22) =7370247168a(23) =22558917120a(24) =68901651456a(25) =210037106688a(26) =639127277568