Number of (w,x,y,z) with all terms in {1,...,n} and |x-y|=|y-z|+1.

Terms

a(0) =0a(1) =0a(2) =4a(3) =18a(4) =56a(5) =120a(6) =228a(7) =378a(8) =592a(9) =864a(10) =1220a(11) =1650a(12) =2184a(13) =2808a(14) =3556a(15) =4410a(16) =5408a(17) =6528a(18) =7812a(19) =9234a(20) =10840a(21) =12600a(22) =14564a(23) =16698a(24) =19056a(25) =21600a(26) =24388a(27) =27378a(28) =30632a(29) =34104