Number of 0..3 arrays x(0..n-1) of n elements with zero n-1st difference.

Terms

a(0) =1a(1) =4a(2) =8a(3) =22a(4) =40a(5) =112a(6) =182a(7) =688a(8) =844a(9) =2090a(10) =2478a(11) =21056a(12) =20762a(13) =112864a(14) =113870a(15) =204796a(16) =141246a(17) =2255872a(18) =1610768a(19) =18946704a(20) =15066098a(21) =20640826a(22) =12378248a(23) =332333696a(24) =279217826a(25) =531539936a(26) =618937930a(27) =1715570198a(28) =330283576a(29) =20131457680