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A241646 Numbers m such that the GCD of the x's that satisfy sigma(x)=m is 1. 5

%I #13 Aug 19 2019 02:59:08

%S 1,12,18,24,31,32,42,48,54,56,60,72,80,84,90,96,98,104,108,114,120,

%T 128,132,140,144,152,156,168,180,182,192,216,224,228,234,240,248,252,

%U 264,270,272,280,288,294,308,312,324,336,342,360,372,384,390,408,420

%N Numbers m such that the GCD of the x's that satisfy sigma(x)=m is 1.

%H Robert Israel, <a href="/A241646/b241646.txt">Table of n, a(n) for n = 1..10000</a>

%e We have sigma(6)=sigma(11)=12, and gcd(6, 11) = 1, hence 12 is in the sequence.

%e For x in [20, 26, 41], sigma(x)=42, and gcd(20, 26, 41)=1, hence 42 is here.

%p N:= 10^4: # for terms <= N

%p V:= Vector(N):

%p for x from 1 to N do

%p s:= numtheory:-sigma(x);

%p if s <= N then

%p if V[s] = 0 then V[s]:= x

%p else V[s]:= igcd(V[s], x)

%p fi

%p fi

%p od: select(t -> V[t]=1, [$1..N]); # _Robert Israel_, Aug 18 2019

%Y Cf. A000203, A240667, A241625, A241646, A241647, A241648, A241649, A241650.

%K nonn

%O 1,2

%A _Michel Marcus_, Apr 26 2014

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Last modified March 29 03:51 EDT 2024. Contains 371264 sequences. (Running on oeis4.)