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 A350038 Numbers that are the perimeter of a primitive 60-degree integer triangle. 3
 18, 20, 35, 36, 45, 56, 77, 84, 90, 104, 110, 120, 126, 135, 143, 170, 176, 182, 189, 198, 209, 210, 216, 221, 252, 260, 264, 266, 270, 272, 273, 297, 299, 323, 350, 351, 360, 368, 374, 378, 380, 390, 396, 425, 432, 437, 459, 462, 464, 468, 476, 494, 495, 506, 527, 551, 561, 570, 575, 585, 594, 608, 612 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Seiichi Manyama, Table of n, a(n) for n = 1..10000 EXAMPLE b(n) = Sum_{k=1..3} A264826(3*n+k-3). c(n) = Sum_{k=1..3} A201223(3*n+k-3). b(1) = c(1) = 3+7+8 = 18 = a(1). b(2) = c(2) = 5+7+8 = 20 = a(2). b(3) = c(5) = 5+19+21 = 45 = a(5). b(4) = c(3) = 7+13+15 = 35 = a(3). b(5) = c(9) = 7+37+40 = 84 = a(8). b(6) = c(4) = 8+13+15 = 36 = a(4). PROG (Ruby) def A(n) ary = [] (1..n).each{|i| (i + 1..n).each{|j| if i.gcd(j) == 1 && (i - j) % 3 > 0 x, y, z = j * j, i * j, i * i ary << 2 * x + 5 * y + 2 * z ary << 3 * x + 3 * y end } } ary end p A(20).uniq.sort[0..100] CROSSREFS Cf. A024364, A201223, A264826, A350013, A350039. Sequence in context: A186781 A164711 A338195 * A335897 A036170 A064271 Adjacent sequences: A350035 A350036 A350037 * A350039 A350040 A350041 KEYWORD nonn AUTHOR Seiichi Manyama, Dec 10 2021 STATUS approved

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Last modified April 16 12:05 EDT 2024. Contains 371711 sequences. (Running on oeis4.)