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%I #19 Feb 09 2021 02:44:15
%S 1,6,12,18,20,24,28,40,56,88,104,120,180,196,224,234,240,360,368,420,
%T 464,496,540,600,650,672,780,992,1080,1344,1504,1872,1888,1890,1952,
%U 2016,2184,2352,2376,2688,3192,3276,3724,3744,4284,4320,4680,5292,5376,5624
%N Nonprime numbers k such that gcd(k, sigma(k)) == sigma(k) (mod k).
%C Complement of primes (A000040) with respect to A205523.
%e 24 is in the sequence because gcd(24; sigma(24)=60) = (sigma(24)=60) mod 24 = 12.
%t Select[Range[10000], ! PrimeQ[#] && Mod[GCD[#, DivisorSigma[1, #]] - DivisorSigma[1, #], #] == 0 &] (* _T. D. Noe_, Feb 03 2012 *)
%o (PARI) isok(k) = if (!isprime(k), my(s=sigma(k)); Mod(gcd(k, s), k) == Mod(s, k)); \\ _Michel Marcus_, Feb 09 2021
%Y Cf. A000203, A000040, A009194, A054024, A205524, A205523.
%K nonn
%O 1,2
%A _Jaroslav Krizek_, Jan 28 2012
%E Corrected by _T. D. Noe_, Feb 03 2012
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