The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 56th year, we are closing in on 350,000 sequences, and we’ve crossed 9,700 citations (which often say “discovered thanks to the OEIS”).

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A205523 Numbers n such that gcd(n, sigma(n)) = sigma(n) (mod n). 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: A059041 A129128 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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified December 6 15:04 EST 2021. Contains 349563 sequences. (Running on oeis4.)