Number of base 4 n-digit numbers with adjacent digits differing by one or less.

Terms

a(0) =1a(1) =4a(2) =10a(3) =26a(4) =68a(5) =178a(6) =466a(7) =1220a(8) =3194a(9) =8362a(10) =21892a(11) =57314a(12) =150050a(13) =392836a(14) =1028458a(15) =2692538a(16) =7049156a(17) =18454930a(18) =48315634a(19) =126491972a(20) =331160282a(21) =866988874a(22) =2269806340a(23) =5942430146a(24) =15557484098a(25) =40730022148a(26) =106632582346a(27) =279167724890a(28) =730870592324