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!)
A158913 Primes p such that there is a composite c with sigma(p) = sigma(c). 8

%I #16 Mar 16 2024 11:18:55

%S 11,17,23,31,41,47,53,59,71,79,83,89,97,103,107,113,127,131,139,151,

%T 167,179,181,191,223,227,233,239,251,263,269,271,293,307,311,359,383,

%U 389,419,431,433,439,443,449,467,479,491,503,521,557,569,571,587,593,599

%N Primes p such that there is a composite c with sigma(p) = sigma(c).

%C See A158914 for the sequence for sigma_2.

%H Donovan Johnson, <a href="/A158913/b158913.txt">Table of n, a(n) for n = 1..10000</a>

%t tp=DivisorSigma[1,Select[Range[1000],PrimeQ]]; tc=DivisorSigma[1,Select[Range[1000],!PrimeQ[ # ]&]]; Intersection[tp,tc]-1

%o (Sage) [sigma(n)-1 for n in (2..600) if is_prime(sigma(n)-1) and n<sigma(n)-1<600] # _Giuseppe Coppoletta_, Dec 22 2014

%K nonn,changed

%O 1,1

%A _T. D. Noe_, Mar 30 2009

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 29 07:27 EDT 2024. Contains 371265 sequences. (Running on oeis4.)