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 A212572 Number of (w,x,y,z) with all terms in {1,...,n} and |w-x| <= |x-y| + |y-z|. 2
 0, 1, 14, 71, 220, 533, 1094, 2015, 3416, 5449, 8270, 12071, 17044, 23421, 31430, 41343, 53424, 67985, 85326, 105799, 129740, 157541, 189574, 226271, 268040, 315353, 368654, 428455, 495236, 569549, 651910, 742911, 843104, 953121, 1073550, 1205063, 1348284 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS For a guide to related sequences, see A211795. LINKS Colin Barker, Table of n, a(n) for n = 0..1000 Index entries for linear recurrences with constant coefficients, signature (3,-1,-5,5,1,-3,1). FORMULA a(n) = 3*a(n-1)-a(n-2)-5*a(n-3)+5*a(n-4)+a(n-5)-3*a(n-6)+a(n-7). From Colin Barker, Dec 06 2015: (Start) a(n) = 1/48*(38*n^4+20*n^3-32*n^2-2*(3*(-1)^n-11)*n+3*((-1)^n-1)). G.f.: x*(1+11*x+30*x^2+26*x^3+9*x^4-x^5) / ((1-x)^5*(1+x)^2). (End) MATHEMATICA t = Compile[{{n, _Integer}}, Module[{s = 0}, (Do[If[Abs[w - x] <= Abs[x - y] + Abs[y - z], s = s + 1], {w, 1, #}, {x, 1, #}, {y, 1, #}, {z, 1, #}] &[n]; s)]]; Map[t[#] &, Range[0, 40]]   (* A212572 *) PROG (PARI) concat(0, Vec(x*(1+11*x+30*x^2+26*x^3+9*x^4-x^5)/((1-x)^5*(1+x)^2) + O(x^100))) \\ Colin Barker, Dec 06 2015 CROSSREFS Cf. A211795, A212570. Sequence in context: A245950 A041372 A245951 * A186707 A212752 A074086 Adjacent sequences:  A212569 A212570 A212571 * A212573 A212574 A212575 KEYWORD nonn,easy AUTHOR Clark Kimberling, May 22 2012 STATUS approved

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Last modified May 18 10:19 EDT 2021. Contains 343995 sequences. (Running on oeis4.)