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

Terms

a(0) =0a(1) =1a(2) =8a(3) =33a(4) =80a(5) =165a(6) =288a(7) =469a(8) =704a(9) =1017a(10) =1400a(11) =1881a(12) =2448a(13) =3133a(14) =3920a(15) =4845a(16) =5888a(17) =7089a(18) =8424a(19) =9937a(20) =11600a(21) =13461a(22) =15488a(23) =17733a(24) =20160a(25) =22825a(26) =25688a(27) =28809a(28) =32144a(29) =35757