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A158913 Primes p such that there is a composite c with sigma(p) = sigma(c). 7
11, 17, 23, 31, 41, 47, 53, 59, 71, 79, 83, 89, 97, 103, 107, 113, 127, 131, 139, 151, 167, 179, 181, 191, 223, 227, 233, 239, 251, 263, 269, 271, 293, 307, 311, 359, 383, 389, 419, 431, 433, 439, 443, 449, 467, 479, 491, 503, 521, 557, 569, 571, 587, 593, 599 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

See A158914 for the sequence for sigma_2.

LINKS

Donovan Johnson, Table of n, a(n) for n = 1..10000

MAPLE

with(numtheory): P:=proc(q) local k, n;

for n from 1 to q do if isprime(n) then for k from 1 to q do

if sigma(n)=sigma(k) then  break; fi; od; if k<n then print(n);

fi; fi; od; end: P(10^9); # Paolo P. Lava, Aug 07 2015

MATHEMATICA

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

PROG

(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

CROSSREFS

Sequence in context: A006621 A337359 A275596 * A066938 A219602 A242260

Adjacent sequences:  A158910 A158911 A158912 * A158914 A158915 A158916

KEYWORD

nonn

AUTHOR

T. D. Noe, Mar 30 2009

STATUS

approved

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Last modified April 17 11:26 EDT 2021. Contains 343064 sequences. (Running on oeis4.)