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A070159 Numbers k such that phi(k)/(sigma(k)-k) is an integer. 3
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 133, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
This sequence consists of all primes p (for which the given ratio equals (p-1)/1, see A000040) and of composites listed in A055940 (see examples).
Up to 10^7, there is no element of this sequence having more than 2 prime factors. - M. F. Hasler, Dec 11 2007
LINKS
Douglas E. Iannucci, On the Equation sigma(n) = n + phi(n), Journal of Integer Sequences, Vol. 20 (2017), Article 17.6.2.
FORMULA
{ a(k) } = { n in N | A000010(n)/A001065(n) is an integer }.
{ a(k) } = { A000040(k) } union { A055940(k) }.
EXAMPLE
The prime p=47 is in this sequence since phi[p]/(sigma[p]-p) = p-1 is an integer, as is the case for any other prime.
The composite n=403=13*31 is in this sequence, since the ratio phi(n)/(sigma[n]-n) =360/(1+13+31)=8 is an integer.
The first few composites in this sequence are 133,403,583,713,... (A055940).
MATHEMATICA
Do[s=EulerPhi[n]/(DivisorSigma[1, n]-n); If[IntegerQ[s], Print[n]], {n, 2, 1000}]
Select[Range[2, 300], IntegerQ[EulerPhi[#]/(DivisorSigma[1, #]-#)]&] (* Harvey P. Dale, Dec 25 2019 *)
PROG
(PARI) for(n=2, 999, eulerphi(n)%(sigma(n)-n) || print1(n", ")) \\ M. F. Hasler, Dec 11 2007
CROSSREFS
Sequence in context: A301378 A052424 A055398 * A322394 A295425 A158611
KEYWORD
nonn
AUTHOR
Labos Elemer, Apr 26 2002
EXTENSIONS
Edited by M. F. Hasler, Dec 11 2007
STATUS
approved

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Last modified March 19 04:58 EDT 2024. Contains 370952 sequences. (Running on oeis4.)