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

Terms

a(0) =0a(1) =1a(2) =6a(3) =23a(4) =52a(5) =105a(6) =178a(7) =287a(8) =424a(9) =609a(10) =830a(11) =1111a(12) =1436a(13) =1833a(14) =2282a(15) =2815a(16) =3408a(17) =4097a(18) =4854a(19) =5719a(20) =6660a(21) =7721a(22) =8866a(23) =10143a(24) =11512a(25) =13025a(26) =14638a(27) =16407a(28) =18284a(29) =20329