Number of -1..1 arrays of n elements with first and second differences also in -1..1.

Terms

a(0) =3a(1) =7a(2) =13a(3) =25a(4) =47a(5) =89a(6) =169a(7) =321a(8) =609a(9) =1155a(10) =2191a(11) =4157a(12) =7887a(13) =14963a(14) =28387a(15) =53855a(16) =102173a(17) =193841a(18) =367751a(19) =697689a(20) =1323641a(21) =2511185a(22) =4764169a(23) =9038483a(24) =17147623a(25) =32532117a(26) =61719263a(27) =117092515a(28) =222145507a(29) =421449879