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 A278648 Consider the set S of integers 1 through n. a(n) is the number of unordered ways in which three distinct elements {a, b, c} of S satisfy a*b = c*n. 3
 0, 0, 0, 0, 0, 0, 2, 0, 2, 1, 4, 0, 8, 0, 6, 8, 7, 0, 13, 0, 16, 12, 10, 0, 26, 6, 12, 13, 24, 0, 38, 0, 23, 20, 16, 24, 46, 0, 18, 24, 50, 0, 56, 0, 40, 49, 22, 0, 71, 15, 46, 32, 48, 0, 67, 40, 74, 36, 28, 0, 120, 0, 30, 73, 61, 48, 92, 0, 64, 44, 106, 0, 136, 0, 36, 86, 72, 60, 110 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,7 COMMENTS Inspired by A278348. Index of first occurrence of k >= 0, or zero if no such number exists: 1, 9, 6, 0, 10, 0, 14, 16, 12, 0, 22, 0, 21, 18, 0, 49, 20, 0, 38, 0, 33, 0, 46, 32, 28, 0, ..., ; Numbers which never occur: 3, 5, 9, 11, 14, 17, 19, 21, 25, 27, 29, 31, 33, 34, 35, 37, 39, 41, 43, 47, 51, ..., ; Records: 0, 2, 4, 8, 13, 16, 26, 38, 46, 50, 56, 71, 74, 120, 136, 176, 193, 214, 330, 355, 482, 574, 668, 839, 890, 996, 1088, 1223, 1528, 1920, 2039, 2224, 2374, 2646, 3055, 3120, 3811, 5010, 5539, 6208, 6591, 8566, 9139, 9690, 12359, 13894, 14796, 15331, 16118, 16558, 22048, ..., ; which first occur for n: 0, 6, 10, 12, 18, 20, 24, 30, 36, 40, 42, 48, 56, 60, 72, 84, 90, 108, 120, 144, 168, 180, 210, 240, 280, 300, 330, 336, 360, 420, 480, 504, 540, 600, 630, 660, 720, 840, 1008, 1080, 1200, 1260, 1440, 1560, 1680, 1980, 2100, 2160, 2310, 2340, 2520, ..., . If instead we look for the number of unordered ways two distinct elements {a, b) of S satisfy a*b = n, then a(n) = floor(sigma_0(n) - 2)) = A211159(n+1). Number of 2 X 2 singular matrices of the form [c a] [b n] with a, b, and c distinct positive integers less than n and a < b. LINKS Bobby Jacobs and Robert G. Wilson v, Table of n, a(n) for n = 0..10000 FORMULA a(n) = (A278348(n) - A278348(n-1))/8. a(p) = 0 for any prime p and for n: 0, 1 & 4. EXAMPLE a(6) = 2 since 2*3 = 1*6 and 3*4 = 2*6; a(8) = 2 since 2*4 = 1*8 and 4*6 = 3*8; a(9) = 1 since 3*6 = 2*9; a(10) = 4 since 2*5 = 1*10, 4*5 = 2*10, 5*6 = 3*10, and 5*8 = 4*10; a(12) = 8 since 2*6 = 1*12, 3*4 = 1*12, 3*8 = 2*12, 4*6 = 2*12, 4*9 = 3*12, 6*8 = 4*12, 6*10 = 5*12, and 8*9 = 6*12; etc. MATHEMATICA f[n_] := Block[{c = 0, k = 1}, While[k < n, c += Count[ Times @@@ Select[ Tuples[ Rest@ Most@ Divisors[k*n], 2], #[[1]] < #[[2]] < n &], k*n]; k++]; c]; Array[f, 52] CROSSREFS Cf. A278348. Sequence in context: A308202 A329152 A337563 * A353508 A029181 A261426 Adjacent sequences: A278645 A278646 A278647 * A278649 A278650 A278651 KEYWORD nonn,easy AUTHOR Bobby Jacobs and Robert G. Wilson v, Nov 25 2016 STATUS approved

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Last modified February 3 02:45 EST 2023. Contains 360024 sequences. (Running on oeis4.)