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A063880
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Numbers n such that sigma(n) = 2*usigma(n).
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4
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108, 540, 756, 1188, 1404, 1836, 2052, 2484, 3132, 3348, 3780, 3996, 4428, 4644, 5076, 5724, 5940, 6372, 6588, 7020, 7236, 7668, 7884, 8316, 8532, 8964, 9180, 9612, 9828, 10260, 10476, 10908, 11124, 11556, 11772, 12204, 12420, 12852, 13716, 14148
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| Numbers so far are all == 108 mod 216. - Ralf Stephan (ralf(AT)ark.in-berlin.de), Jul 07 2003. Confirmed up to 10^7 by Robert G. Wilson v.
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LINKS
| Harry J. Smith, Table of n, a(n) for n=1,...,1000
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MATHEMATICA
| usigma[n_] := Block[{d = Divisors[n]}, Plus @@ Select[d, GCD[ #, n/# ] == 1 &]]; Select[ Range[ 14363], DivisorSigma[1, # ] == 2usigma[ # ] &] (from Robert G. Wilson v Aug 28 2004)
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PROG
| (PARI) u(n) = sumdiv(n, d, if(gcd(d, n/d)==1, d)); j=[]; for(n=1, 30000, if(sigma(n)-u(n)==u(n), j=concat(j, n))); j
(PARI) u(n) = sumdiv(n, d, if(gcd(d, n/d)==1, d)) { n=0; for (m=1, 10^9, if(sigma(m) == 2*u(m), write("b063880.txt", n++, " ", m); if (n==1000, break)) ) } [From Harry J. Smith (hjsmithh(AT)sbcglobal.net), Sep 01 2009]
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CROSSREFS
| Cf. A000203, A048146, A034448, A097702.
Sequence in context: A192793 A202435 A202428 * A059436 A160919 A129027
Adjacent sequences: A063877 A063878 A063879 * A063881 A063882 A063883
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KEYWORD
| easy,nonn
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AUTHOR
| Jason Earls (zevi_35711(AT)yahoo.com), Aug 27 2001
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