Number of length-n ternary strings that do not contain both "00" and "11".

Terms

a(0) =1a(1) =3a(2) =9a(3) =27a(4) =79a(5) =229a(6) =657a(7) =1871a(8) =5295a(9) =14909a(10) =41801a(11) =116783a(12) =325287a(13) =903741a(14) =2505377a(15) =6932479a(16) =19151519a(17) =52833853a(18) =145578265a(19) =400705135a(20) =1101936119a(21) =3027902045a(22) =8314284721a(23) =22816209855a(24) =62579270191a(25) =171559358493a(26) =470132335209