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 A088658 Number of triangles in an n X n unit grid that have minimal possible area (of 1/2). 10
 0, 4, 32, 124, 320, 716, 1328, 2340, 3792, 5852, 8544, 12260, 16864, 22916, 30272, 39188, 49824, 62948, 78080, 96348, 117232, 141260, 168480, 200292, 235680, 276100, 321056, 371484, 427024, 489900, 558112, 634724, 718432, 810116, 909600, 1018388, 1135136, 1263828, 1402304, 1551908 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Ray Chandler, Table of n, a(n) for n = 1..1000 N. J. A. Sloane, Families of Essentially Identical Sequences, Mar 24 2021 (Includes this sequence) FORMULA a(n+1) = 4*A115004(n). EXAMPLE a(2)=4 because 4 (isosceles right) triangles with area 1/2 can be placed on a 2 X 2 grid. MATHEMATICA z[n_] := Sum[(n - i + 1)(n - j + 1) Boole[GCD[i, j] == 1], {i, n}, {j, n}]; a[n_] := 4 z[n - 1]; Array[a, 40] (* Jean-François Alcover, Mar 24 2020 *) CROSSREFS Cf. A045996. The following eight sequences are all essentially the same. The simplest is A115004(n), which we denote by z(n). Then A088658(n) = 4*z(n-1); A114043(n) = 2*z(n-1)+2*n^2-2*n+1; A114146(n) = 2*A114043(n); A115005(n) = z(n-1)+n*(n-1); A141255(n) = 2*z(n-1)+2*n*(n-1); A290131(n) = z(n-1)+(n-1)^2; A306302(n) = z(n)+n^2+2*n. - N. J. A. Sloane, Feb 04 2020 Sequence in context: A267668 A338322 A239056 * A088802 A123854 A301843 Adjacent sequences:  A088655 A088656 A088657 * A088659 A088660 A088661 KEYWORD nonn AUTHOR Yuval Dekel (dekelyuval(AT)hotmail.com), Nov 21 2003 EXTENSIONS a(7)-a(28) from Ray Chandler, May 03 2011 Corrected and extended by Ray Chandler, May 18 2011 STATUS approved

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Last modified June 19 03:23 EDT 2021. Contains 345125 sequences. (Running on oeis4.)