login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A091286 Numbers n such that sigma_3(n) is divisible by square of cototient of n, while n is not a prime number. 1
15, 85, 259, 295, 391, 589, 799, 1111, 1717, 3193, 4171, 4369, 12361, 17473, 23533, 25429, 28243, 31351, 34477, 36181, 41407, 47989, 51143, 52537, 58103, 65641, 68377, 69541, 69919, 70453, 72619, 81121, 83131, 83767, 85069, 91759 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
LINKS
EXAMPLE
n=15: cototient(15) = 7, sigma_3(15) = 3528 = 72 * 49.
MATHEMATICA
Do[s=DivisorSigma[3, n]/(n-EulerPhi[n])^2; If[IntegerQ[s]&&!PrimeQ[n], Print[n]], {n, 1, 100000}]
PROG
(PARI) isok(n) = (n!=1) && !isprime(n) && !(sigma(n, 3)%(n-eulerphi(n))^2); \\ Michel Marcus, Aug 13 2019
CROSSREFS
Sequence in context: A067401 A206169 A160599 * A176070 A160747 A064058
KEYWORD
nonn
AUTHOR
Labos Elemer, Feb 03 2004
STATUS
approved

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

License Agreements, Terms of Use, Privacy Policy. .

Last modified March 19 04:26 EDT 2024. Contains 370952 sequences. (Running on oeis4.)