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 A135470 Composite numbers n that divide 2 * sigma(n) - d(n) [that is, 2 * sum of divisors - number of divisors]. 1
 18, 70, 88, 132, 780, 11096, 17816, 518656, 1713592, 9928792, 11547352, 13499120, 17999992, 89283592, 134094848, 173482552, 175519344, 318836720, 903230984, 1081850752, 2146926592, 16709332608, 18543186000, 34177937500 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS It is conjectured that there are no odd terms and that the sequence is infinite. a(25) > 10^12. - Donovan Johnson, Nov 15 2013 LINKS Yong-Gao Chen and Jin-Hui Fang, On n phi(n)d(n) + 2 and n phi(n) sigma(n)+1, INTEGERS: Electronic J. Combinatorial Number Theory: 8 (2008), #A7. Donovan Johnson, 10 terms > 10^12 MATHEMATICA Select[Range[172*10^4], CompositeQ[#]&&Divisible[2*DivisorSigma[1, #]-DivisorSigma[ 0, #], #]&] (* The program generates the first 9 terms of the sequence. To generate more, increase the Range constant. *) (* Harvey P. Dale, May 11 2020 *) PROG (PARI) isok(n) = (! isprime(n) && (2*sigma(n) - numdiv(n)) % n == 0) \\ Michel Marcus, Jun 09 2013 CROSSREFS Sequence in context: A158056 A304061 A214491 * A059224 A174492 A088490 Adjacent sequences:  A135467 A135468 A135469 * A135471 A135472 A135473 KEYWORD nonn AUTHOR R. K. Guy, Feb 08 2008 EXTENSIONS Corrected by Emeric Deutsch, Feb 29 2008 a(15)-a(24) from Donovan Johnson, Oct 13 2008 STATUS approved

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Last modified June 7 05:20 EDT 2020. Contains 334837 sequences. (Running on oeis4.)