Number of zero-sum -2..2 arrays of n elements with first through third differences also in -2..2.

Terms

a(0) =1a(1) =3a(2) =5a(3) =15a(4) =21a(5) =53a(6) =109a(7) =239a(8) =489a(9) =1051a(10) =2253a(11) =4853a(12) =10511a(13) =23033a(14) =50791a(15) =112187a(16) =248217a(17) =549917a(18) =1220585a(19) =2712351a(20) =6037825a(21) =13459637a(22) =30045375a(23) =67137267a(24) =150165265a(25) =336151499a(26) =753099801a(27) =1688448381a(28) =3788174371a(29) =8504582261