|
%I M4827 #24 May 08 2018 15:11:54
%S 12,18,31,32,54,56,80,98,104,108,114,124,126,128,132,140,152,156,182,
%T 186,210,264,272,280,308,320,342,378,390,392,399,403,408,416,440,444,
%U 448,492,522,532,570,572,594,608,630,632,726,762,770,774,780,784,800
%N Numbers n such that sigma(x) = n has exactly 2 solutions.
%D M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, 1964 (and various reprintings), p. 840.
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H Donovan Johnson, <a href="/A007371/b007371.txt">Table of n, a(n) for n = 1..1000</a>
%H M. Abramowitz and I. A. Stegun, eds., <a href="http://www.convertit.com/Go/ConvertIt/Reference/AMS55.ASP">Handbook of Mathematical Functions</a>, National Bureau of Standards, Applied Math. Series 55, Tenth Printing, 1972 [alternative scanned copy].
%H R. G. Wilson, V, <a href="/A007015/a007015.pdf">Letter to N. J. A. Sloane, Jul. 1992</a>
%t a = Table[ 0, {750} ]; Do[ s = DivisorSigma[ 1, n ]; If[ s < 751, a[ [ s ] ]++ ], {n, 1, 750} ]; Select[ Range[ 750 ], a[ [ # ] ] == 2 & ]
%o (PARI) is(n)=sum(k=1,n,sigma(k)==n)==2 \\ _Charles R Greathouse IV_, Mar 09 2014
%Y Cf. A007369, A007370, A007372, etc.
%K nonn
%O 1,1
%A _N. J. A. Sloane_, _Mira Bernstein_, _Robert G. Wilson v_
|
