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 A291485 Numbers m such that sigma(x) = m*(m+1)/2 has at least one solution. 0
 1, 2, 3, 5, 7, 8, 12, 13, 15, 18, 20, 24, 27, 30, 31, 32, 35, 38, 39, 47, 48, 51, 55, 56, 62, 63, 64, 79, 80, 84, 90, 92, 95, 96, 104, 111, 116, 119, 120, 128, 135, 140, 142, 143, 144, 147, 152, 155, 156, 159, 160, 167, 168, 170, 171, 175, 176, 182, 184, 188, 191, 192, 195, 203, 207, 208 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Let b(n) be the smallest k such that sigma(k) is the n-th triangular number, or 0 if no such k exists. For n >= 1, b(n) sequence is 1, 2, 5, 0, 8, 0, 12, 22, 0, 0, 0, 45, 36, 0, 54, 0, 0, 98, 0, 104, 0, 0, 0, 152, 0, 0, 160, 0, 0, 200, ... LINKS EXAMPLE 15 is a term because sigma(54) = sigma(56) = sigma(87) = sigma(95) = A000217(15). MAPLE N:= 1000: # to get all terms <= N Sigmas:= {seq(numtheory:-sigma(x), x=1..N*(N+1)/2)}: select(t -> member(t*(t+1)/2, Sigmas), [\$1..N]); # Robert Israel, Aug 25 2017 MATHEMATICA invT[n_] := (Sqrt[8*n+1]-1)/2; Union@ Select[invT /@ DivisorSigma[1, Range[ 208*209/2]], IntegerQ[#] && # <= 208 &] (* Giovanni Resta, Aug 25 2017 *) CROSSREFS Cf. A000203, A000217, A045746. Sequence in context: A224858 A090704 A112055 * A228898 A076386 A177801 Adjacent sequences:  A291482 A291483 A291484 * A291486 A291487 A291488 KEYWORD nonn,easy AUTHOR Altug Alkan, Aug 24 2017 STATUS approved

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Last modified September 20 05:17 EDT 2021. Contains 347577 sequences. (Running on oeis4.)