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) =14a(3) =71a(4) =220a(5) =533a(6) =1094a(7) =2015a(8) =3416a(9) =5449a(10) =8270a(11) =12071a(12) =17044a(13) =23421a(14) =31430a(15) =41343a(16) =53424a(17) =67985a(18) =85326a(19) =105799a(20) =129740a(21) =157541a(22) =189574a(23) =226271a(24) =268040a(25) =315353a(26) =368654a(27) =428455a(28) =495236a(29) =569549