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A060664 Sigma(x) = n has exactly eight solutions. 1

%I

%S 336,432,672,756,840,1536,1620,1764,1848,2280,2394,2604,2808,3264,

%T 4080,4480,4860,5328,6528,6624,7128,8316,8568,8880,10608,11040,11448,

%U 12288,12420,12636,13176,13200,13248,13536,13860,14196,14208,14448,14700

%N Sigma(x) = n has exactly eight solutions.

%H Robert Israel, <a href="/A060664/b060664.txt">Table of n, a(n) for n = 1..10000</a>

%e 336 = sigma(132) = sigma(140) = sigma(182) = sigma(188) = sigma(195) = sigma(249) = sigma(287) = sigma(299).

%p N:= 30000: # to get terms <= N

%p V:= Vector(N):

%p for k from 1 to N-1 do

%p t:= numtheory:-sigma(k);

%p if t <= N then V[t]:= V[t]+1 fi

%p od:

%p select(t -> V[t]=8, [$1..N]); # _Robert Israel_, Sep 22 2019

%t a = Table[ 0, {20000} ]; Do[ s = DivisorSigma[ 1, n ]; If[ s < 20001, a[ [ s ] ]++ ], {n, 1, 20000} ]; Select[ Range[ 20000 ], a[ [ # ] ] == 8 & ]

%Y Cf. A007370, A007371, A007372.

%K nonn

%O 1,1

%A _Robert G. Wilson v_, Apr 18 2001

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Last modified November 19 03:42 EST 2019. Contains 329310 sequences. (Running on oeis4.)