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

Terms

a(0) =1a(1) =1a(2) =3a(3) =3a(4) =5a(5) =11a(6) =21a(7) =39a(8) =73a(9) =129a(10) =229a(11) =407a(12) =733a(13) =1335a(14) =2461a(15) =4549a(16) =8427a(17) =15585a(18) =28811a(19) =53235a(20) =98479a(21) =182403a(22) =338461a(23) =628873a(24) =1169875a(25) =2177875a(26) =4056997a(27) =7561019a(28) =14098931a(29) =26303445