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

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A205597 Odd terms of A019278: odd n such that sigma(sigma(n))/n is an integer. 2
 1, 15, 21, 1023, 29127, 550095, 355744082763 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS a(8) > 4*10^12, if it exists. - Giovanni Resta, Feb 26 2020 First five terms are squarefree. Sigma(sigma(n))/n ratios for these seven known terms are: 1, 4, 3, 4, 4, 6, 4. - Antti Karttunen, Mar 19 2021 LINKS EXAMPLE 15 is odd, sigma(15) = 24, sigma(24) = 60, and 60/15 is integer. MATHEMATICA Select[Range[1, 10^6, 2], Mod[Nest[DivisorSigma[1, #] &, #, 2], #] == 0 &] (* Michael De Vlieger, Mar 19 2021 *) PROG (PARI) isok(n) = (n%2) && (denominator(sigma(sigma(n))/n) == 1); \\ Michel Marcus, Sep 27 2017 CROSSREFS Cf. A000203, A019278, A019283, A228058, A323653. Sequence in context: A084931 A265153 A219214 * A307792 A236764 A300958 Adjacent sequences:  A205594 A205595 A205596 * A205598 A205599 A205600 KEYWORD nonn,more AUTHOR Jud McCranie, Feb 08 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 April 16 04:36 EDT 2021. Contains 343030 sequences. (Running on oeis4.)