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 A024164 Number of integer-sided triangles with sides a,b,c, a
 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 0, 2, 0, 0, 2, 0, 0, 3, 0, 0, 3, 0, 0, 4, 0, 0, 4, 0, 0, 5, 0, 0, 5, 0, 0, 6, 0, 0, 6, 0, 0, 7, 0, 0, 7, 0, 0, 8, 0, 0, 8, 0, 0, 9, 0, 0, 9, 0, 0, 10, 0, 0, 10, 0, 0, 11, 0, 0, 11, 0, 0, 12, 0, 0, 12, 0, 0, 13, 0, 0, 13, 0, 0, 14, 0, 0, 14, 0, 0, 15, 0, 0, 15, 0, 0, 16, 0, 0, 16 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,15 COMMENTS From Bernard Schott, Oct 10 2020: (Start) Equivalently: number of integer-sided triangles whose sides a < b < c are in arithmetic progression with perimeter n. Equivalently: number of integer-sided triangles such that b = (a+c)/2 with a < c and perimeter n. All the perimeters are multiple of 3 because each perimeter = 3 * middle side b. For each perimeter n = 12*k with k>0, there exists one and only one such right integer triangle whose triple is (3k, 4k, 5k). For the corresponding primitive triples and miscellaneous properties and references, see A336750. (End) LINKS Wikipedia, Integer Triangle Index entries for linear recurrences with constant coefficients, signature (0,0,1,0,0,1,0,0,-1). FORMULA If n = 3*k, then a(n) = floor((n-3)/6) = A004526((n-3)/3), otherwise, a(3k+1) = a(3k+2) = 0. - Bernard Schott, Oct 10 2020 From Wesley Ivan Hurt, Nov 01 2020: (Start) G.f.: (-1 + x^3 + x^6)/((x^3 - 1)^2*(x^3 + 1)). a(n) = a(n-3) + a(n-6) - a(n-9). a(n) = (1 - ceiling(n/3) + floor(n/3)) * floor((n-3)/6). (End) EXAMPLE a(9) = 1 for the smallest such triangle (2, 3, 4). a(12) = 1 for Pythagorean triple (3, 4, 5). a(15) = 2 for the two triples (3, 5, 7) and (4, 5, 6). MATHEMATICA A024164[n_] := If[Mod[n, 3] == 0, Floor[(n - 3)/6], 0]; Array[A024164, 100] (* Wesley Ivan Hurt, Nov 01 2020 *) LinearRecurrence[{0, 0, 1, 0, 0, 1, 0, 0, -1}, {0, 0, 0, 0, 0, 0, 0, 0, 1}, 120] (* Harvey P. Dale, Jun 03 2021 *) CROSSREFS Cf. A336750 (triples), A336751 (smallest side), A307136 (middle side), A336753 (largest side), A336754 (perimeter), this sequence (number of triangles whose perimeter = n), A336755 (primitive triples), A336756 (primitive perimeters), A336757 (number of primitive triangles with perimeter = n). Cf. A005044 (number of integer-sided triangles with perimeter = n). Sequence in context: A033719 A171608 A307985 * A138805 A316400 A061897 Adjacent sequences:  A024161 A024162 A024163 * A024165 A024166 A024167 KEYWORD nonn,easy AUTHOR STATUS approved

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Last modified May 25 13:00 EDT 2022. Contains 354071 sequences. (Running on oeis4.)