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 A027687 4-perfect (quadruply-perfect or sous-triple) numbers: sum of divisors of n is 4n. 27
 30240, 32760, 2178540, 23569920, 45532800, 142990848, 1379454720, 43861478400, 66433720320, 153003540480, 403031236608, 704575228896, 181742883469056, 6088728021160320, 14942123276641920, 20158185857531904, 275502900594021408 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS It is conjectured that there are only finitely many terms. - N. J. A. Sloane, Jul 22 2012 Odd perfect number (unlikely to exist) and infinitely many Mersenne primes will make the sequence infinite - take the product of the OPN and coprime EPNs. Conjecture: A010888(a(n)) divides a(n). Tested for n up to 36 incl. - Ivan N. Ianakiev, Oct 31 2013 From Farideh Firoozbakht, Dec 26 2014: (Start) Theorem: If k>1 and p=a(n)/2^(k-2)+1 is prime then for each positive integer m, 2^(k-1)*p^m is a solution to the equation sigma(phi(x))=2*x-2^k, which implies the equation has infinitely many solutions. Proof: sigma(phi(2^(k-1)*p^m)) = sigma(2^(k-2)*(p-1)*p^(m-1)) = sigma(2^(k-2)*(p-1))*sigma(p^(m-1)) = sigma(a(n))*(p^m-1)/(p-1) = 4*a(n)*(p^m-1)/(p-1) = 2^k*(p^m-1) = 2*(2^(k-1)*p^m)-2^k. It seems that for all such equations there exist such an infinite set of solutions. So I conjecture that the sequence is infinite! (End) If 3 were prepended to this sequence, then it would be the sequence of integers k such that numerator(sigma(k)/k)=4. - Michel Marcus, Nov 22 2015 REFERENCES R. K. Guy, Unsolved Problems in Number Theory, B2. LINKS T. D. Noe, Table of n, a(n) for n=1..36 (complete sequence from Flammenkamp) Abiodun E. Adeyemi, A Study of @-numbers, arXiv:1906.05798 [math.NT], 2019. K. A. Broughan, Qizhi Zhou, Divisibility by 3 of even multiperfect numbers of abundancy 3 and 4, JIS 13 (2010) 10.1.5 F. Firoozbakht, M. F. Hasler, Variations on Euclid's formula for Perfect Numbers, JIS 13 (2010) #10.3.1. Achim Flammenkamp, The Multiply Perfect Numbers Page Fred Helenius, Link to Glossary and Lists Walter Nissen, Abundancy : Some Resources Eric Weisstein's World of Mathematics, Multiperfect Number. Eric Weisstein's World of Mathematics, Sous-Triple. Wikipedia, Multiply perfect number EXAMPLE From Daniel Forgues, May 09 2010: (Start) 30240 = 2^5*3^3*5*7 sigma(30240) = (2^6-1)/1*(3^4-1)/2*(5^2-1)/4*(7^2-1)/6 = (63)*(40)*(6)*(8) = (7*3^2)*(2^3*5)*(2*3)*(2^3) = 2^7*3^3*5*7 = (2^2) * (2^5*3^3*5*7) = 4 * 30240 (End) MATHEMATICA AbundantQ[n_]:=DivisorSigma[1, n]==4*n; a={}; Do[If[AbundantQ[n], AppendTo[a, n]], {n, 10^6}]; a (* Vladimir Joseph Stephan Orlovsky, Aug 16 2008 *) CROSSREFS Cf. A000396, A005820, A007539, A046060, A046061. Sequence in context: A254841 A247855 A113286 * A190475 A218029 A323991 Adjacent sequences:  A027684 A027685 A027686 * A027688 A027689 A027690 KEYWORD nonn AUTHOR Jean-Yves Perrier (nperrj(AT)ascom.ch) EXTENSIONS 4 more terms from Labos Elemer STATUS approved

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Last modified October 15 22:25 EDT 2019. Contains 328038 sequences. (Running on oeis4.)