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 A007372 Numbers n such that sigma(x) = n has exactly 3 solutions. (Formerly M5142) 14
 24, 42, 48, 60, 84, 90, 224, 228, 234, 248, 270, 294, 324, 450, 468, 528, 558, 620, 640, 660, 810, 882, 888, 896, 968, 972, 1020, 1050, 1104, 1116, 1140, 1216, 1232, 1240, 1274, 1332, 1392, 1400, 1452, 1456, 1464, 1482, 1524, 1530, 1600, 1694, 1716, 1760 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 REFERENCES M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, 1964 (and various reprintings), p. 840. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Donovan Johnson, Table of n, a(n) for n = 1..1000 M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards, Applied Math. Series 55, Tenth Printing, 1972 [alternative scanned copy]. R. G. Wilson, V, Letter to N. J. A. Sloane, Jul. 1992 MATHEMATICA a = Table[ 0, {2500} ]; Do[ s = DivisorSigma[ 1, n ]; If[ s < 2501, a[ [ s ] ]++ ], {n, 1, 2500} ]; Select[ Range[ 2500 ], a[ [ # ] ] == 3 & ] PROG (PARI) is(n)=sum(k=1, n, sigma(k)==n)==3 \\ Charles R Greathouse IV, Mar 09 2014 CROSSREFS Cf. A007369, A007370, A007371, etc. Sequence in context: A304890 A264155 A241254 * A062910 A098900 A098769 Adjacent sequences:  A007369 A007370 A007371 * A007373 A007374 A007375 KEYWORD nonn AUTHOR STATUS approved

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Last modified October 18 04:57 EDT 2019. Contains 328145 sequences. (Running on oeis4.)