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A175850
Numbers m with property that m-th triangular number is a sum of divisors of some k-th triangular number (A175849).
2
1, 13, 12, 384, 575, 783, 4095, 4607, 4095, 6912, 12543, 13824, 16895, 21504, 20735, 27264, 40959, 68256, 76544, 104832, 175104, 130559, 146432, 180224, 129024, 202239, 316224, 328320, 372735, 395199, 512000, 532575, 512000, 732159, 787968, 1181439, 1756160, 2253824
OFFSET
1,2
LINKS
FORMULA
sigma(T(k)) = T(m); A000203(A000217(k)) = A000217(m).
EXAMPLE
Some pairs of k,m: 1,1; 8,13; 9,12; 215,384; 458,575; 520,783; 2232,4095; 3251,4607; 3634,4095; 5349,6912; 9489,12543; 10051,13824.
MATHEMATICA
f[n_] := Sqrt[8*DivisorSigma[1, n*(n+1)/2] + 1]; (f /@ Select[Range[10^4], IntegerQ @ f[#] &] - 1)/2 (* Amiram Eldar, Feb 23 2020 *)
PROG
(PARI) {for(n=1, 10^7, m=sigma(n*(n+1)/2); issquare(d=1+8*m) && print1((sqrtint(d)-1)/2, ", "))} \\ edited by Michel Marcus, Feb 23 2020
CROSSREFS
Cf. A000203 (sigma(n) = sum of divisors of n), A000217 (triangular numbers), A175849 (corresponding values of n).
Sequence in context: A113548 A249998 A195210 * A176306 A291368 A051392
KEYWORD
nonn
AUTHOR
Zak Seidov, Sep 27 2010
EXTENSIONS
Data corrected and extended by Amiram Eldar, Feb 23 2020
STATUS
approved