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 A205523 Numbers k such that gcd(k, sigma(k)) == sigma(k) (mod k). 6
 1, 2, 3, 5, 6, 7, 11, 12, 13, 17, 18, 19, 20, 23, 24, 28, 29, 31, 37, 40, 41, 43, 47, 53, 56, 59, 61, 67, 71, 73, 79, 83, 88, 89, 97, 101, 103, 104, 107, 109, 113, 120, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 180, 181, 191, 193, 196, 197, 199 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Numbers m such that A009194(m) = gcd(m, A000203(m)) = A000203(m) mod m = A054024(m). Complement of A205524. Union of primes (A000040) and composite numbers from A205525. LINKS Ivan Neretin, Table of n, a(n) for n = 1..10000 EXAMPLE Number 24 is in sequence because gcd(24, sigma(24)) = (sigma(24)=60) mod 24 = 12. MATHEMATICA Select[Range[300], Mod[GCD[#, DivisorSigma[1, #]] - DivisorSigma[1, #], #] == 0 &] PROG (PARI) isok(n) = (gcd(n, sigma(n)) % n) == (sigma(n) % n); \\ Michel Marcus, Dec 22 2017 CROSSREFS Cf. A000203, A000040, A009194, A054024, A205524, A205525. Sequence in context: A355647 A367585 A164922 * A343027 A145739 A198191 Adjacent sequences: A205520 A205521 A205522 * A205524 A205525 A205526 KEYWORD nonn AUTHOR Jaroslav Krizek, Jan 28 2012 EXTENSIONS Corrected by T. D. Noe, Feb 03 2012 STATUS approved

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Last modified September 15 22:11 EDT 2024. Contains 375959 sequences. (Running on oeis4.)