Number of -2..2 arrays of n elements with first, second and third differences also in -2..2.

Terms

a(0) =5a(1) =19a(2) =57a(3) =127a(4) =293a(5) =663a(6) =1517a(7) =3459a(8) =7905a(9) =18051a(10) =41239a(11) =94203a(12) =215217a(13) =491661a(14) =1123219a(15) =2566023a(16) =5862189a(17) =13392389a(18) =30595439a(19) =69896451a(20) =159681185a(21) =364797903a(22) =833395099a(23) =1903923685a(24) =4349588175a(25) =9936804401a(26) =22701018621a(27) =51861365563