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A066764 Numbers n such that (n, sigma(n)) lies on a circle with integral radius centered at the origin, i.e., n^2 + sigma(n)^2 is a square. 2
3, 8, 30, 140, 315, 319, 460, 840, 864, 936, 1040, 1287, 1836, 2480, 2600, 6076, 6200, 7788, 11398, 12035, 12415, 12584, 13260, 13734, 15886, 18480, 26078, 33820, 40640, 42665, 46200, 55860, 68352, 70266, 70645, 75330, 78939, 81740, 98230 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Also numbers such that A017665(n)^2 + A017666(n)^2 is a square. - Michel Marcus, May 23 2015

LINKS

Harry J. Smith, Table of n, a(n) for n = 1..731

EXAMPLE

3^2 + sigma(3)^2 = 9 + 16 = 5^2; so 3 is a term of the sequence.

MATHEMATICA

Select[ Range[ 1, 10^5 ], IntegerQ[ Sqrt[ #^2 + DivisorSigma[ 1, # ]^2 ] ] & ]

PROG

(PARI) { n=0; for (m=1, 10^10, if (issquare(m^2 + sigma(m)^2), write("b066764.txt", n++, " ", m); if (n==1000, return)) ) } \\ Harry J. Smith, Mar 24 2010

CROSSREFS

Sequence in context: A161779 A074501 A009123 * A059171 A261766 A078619

Adjacent sequences:  A066761 A066762 A066763 * A066765 A066766 A066767

KEYWORD

nonn

AUTHOR

Joseph L. Pe, Jan 17 2002

STATUS

approved

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Last modified February 20 21:41 EST 2018. Contains 299387 sequences. (Running on oeis4.)