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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

Table of n, a(n) for n=1..24.

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

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 May 24 18:34 EDT 2019. Contains 323534 sequences. (Running on oeis4.)