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A095301 Numbers n such that there is some k < n with n*sigma(k) = k*sigma(n). 8
28, 140, 200, 224, 234, 270, 308, 364, 476, 496, 532, 600, 644, 672, 700, 812, 819, 868, 936, 1036, 1148, 1170, 1204, 1316, 1400, 1484, 1488, 1540, 1638, 1652, 1708, 1800, 1820, 1876, 1988, 2016, 2044, 2200, 2212, 2324, 2380, 2464, 2480, 2492, 2574, 2600 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Original name: Numbers n such that A094759(n) < n.

Agrees with A050973 without duplicates.

Also numbers n such that the value sigma(n)/n has already been reached before n. If n belongs to the sequence then A214701(n) = A214701(n-1). - Michel Marcus, Aug 19 2012

REFERENCES

B. Spearman and K. S. Williams, Handbook of Estimates in the Theory of Numbers, Carleton Math. Lecture Note Series No. 14, 1975; see p. 3.2, Eq. (3.9).

LINKS

Michel Marcus and Donovan Johnson, Table of n, a(n) for n = 1..10000 (first 180 terms from Michel Marcus)

P. Erdős, Remarks on number theory II: Some problems on the sigma function, Acta Arith., 5 (1959), 171-177.

EXAMPLE

A094759(28) = 6 < 28, hence 28 is in the sequence.

PROG

(PARI) for(n=1, 2600, s=sigma(n); k=1; while(n*sigma(k)!=k*s, k++); if(k<n, print1(n, ", ")));

(PARI) allab = []; nb = 0; for (i=1, n, ab = sigma(i)/i; already = 0; if (length(allab) > 0, for (j=1, length(allab), if (ab == allab[j], already = 1; break); ); ); if (already == 1,  nb++; print1(i, ", "), allab = concat(allab, ab); ); )

/* Michel Marcus, Aug 19 2012 */

CROSSREFS

Cf. A000203, A094759, A050973.

Sequence in context: A201095 A239205 A050973 * A201102 A317680 A042530

Adjacent sequences:  A095298 A095299 A095300 * A095302 A095303 A095304

KEYWORD

nonn

AUTHOR

Klaus Brockhaus, Jun 01 2004

EXTENSIONS

New name from Charles R Greathouse IV, Jul 21 2015

STATUS

approved

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Last modified August 2 02:30 EDT 2021. Contains 346409 sequences. (Running on oeis4.)