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 A317681 a(n) = smallest m such that sigma(m) = n*m/2. 0
 1, 2, 6, 24, 120, 4320, 30240, 8910720, 14182439040, 17116004505600, 154345556085770649600, 170974031122008628879954060917200710847692800, 141310897947438348259849402738485523264343544818565120000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,2 COMMENTS Interleaving of A007539 and A088912. For even n, a(n) is a multiply perfect number; for odd n it is a hemiperfect number. Note that 1 is the only number with abundancy 1 and 2 is the only number with abundancy 3/2. For k >= 3 it is not known whether there are no or finitely many or infinitely many numbers with abundancy k/2. Also it is not known whether a(n) < a(n+1) always holds. On the Riemann Hypothesis (RH), a(n) > exp(exp(n/(2*exp(gamma)))), where gamma = 0.5772156649... is the Euler-Mascheroni constant (A001620). LINKS Achim Flammenkamp, The Multiply Perfect Numbers Page Fred Helenius, Link to Glossary and Lists G. P. Michon, Multiplicative functions: Abundancy = sigma(n)/n G. P. Michon, Multiperfect and hemiperfect numbers Walter Nissen, Abundancy: Some Resources FORMULA a(2n) = A007539(n), a(2n+1) = A088912(n), n > 0. EXAMPLE a(7) = 4320 since sigma(4320) = 15120 = 7/2*4320 and 4320 is the smallest m such that sigma(m)/m = 7/2. MATHEMATICA Nest[Append[#, Block[{m = #1[[-1]] + 1}, While[DivisorSigma[1, m] != #2 m/2, m++]; m]] & @@ {#, Length@ # + 2} &, {1}, 6] (* Michael De Vlieger, Aug 05 2018 *) PROG (PARI) for(n=2, 10, for(m=1, 10^12, if(sigma(m)/m==n/2, print1(m, ", "); break()))) CROSSREFS Cf. A007539, A088912, A246454. Numbers with abundancy k/2: A000396 (k=4), A141643 (k=5), A005820 (k=6), A055153 (k=7), A027687 (k=8), A141645 (k=9), A046060 (k=10), A159271 (k=11), A046061 (k=12), A160678 (k=13). Sequence in context: A037992 A335386 A114779 * A230363 A275753 A144167 Adjacent sequences:  A317678 A317679 A317680 * A317682 A317683 A317684 KEYWORD nonn,hard,more AUTHOR Jianing Song, Aug 04 2018 STATUS approved

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Last modified June 12 14:32 EDT 2021. Contains 344957 sequences. (Running on oeis4.)