Number of 0..5 arrays x(0..n-1) of n elements with zero n-1st difference.

Terms

a(0) =1a(1) =6a(2) =18a(3) =66a(4) =202a(5) =730a(6) =2036a(7) =7488a(8) =19262a(9) =62772a(10) =190408a(11) =1077600a(12) =2185196a(13) =12081408a(14) =21927244a(15) =52980770a(16) =148851716a(17) =1353060240a(18) =2585159746a(19) =16942421808a(20) =28793485330a(21) =51092435876a(22) =106822624510