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A248792 Numbers n such that sigma(n) - 1 is a prime p. 7
2, 3, 5, 6, 7, 10, 11, 13, 14, 15, 17, 19, 20, 21, 23, 24, 26, 29, 30, 31, 33, 34, 35, 37, 38, 40, 41, 43, 44, 46, 47, 51, 52, 53, 55, 57, 58, 59, 60, 61, 63, 65, 67, 71, 73, 74, 76, 78, 79, 83, 84, 85, 86, 88, 89, 90, 92, 93, 96, 97, 101, 103, 105, 107, 109 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Union of primes (A000040) and terms of A066073 (composites).

Numbers n such that A039653(n) is prime.

Corresponding values of primes p are in A248793.

LINKS

Michael De Vlieger, Table of n, a(n) for n = 1..10000

OEIS Wiki, Cyclotomic Polynomials at x=n, n! and sigma(n)

EXAMPLE

6 is in sequence because sigma(6) - 1 = 12 - 1 = 11 (prime).

MAPLE

with(numtheory): A248792:=n->`if`(isprime(sigma(n)-1), n, NULL): seq(A248792(n), n=1..200); # Wesley Ivan Hurt, Jul 09 2015

MATHEMATICA

Select[Range[110], PrimeQ[DivisorSigma[1, #] - 1] &] (* Vincenzo Librandi, Nov 02 2014 *)

PROG

(MAGMA) [n: n in[1..1000] | IsPrime(SumOfDivisors(n) - 1)]

(PARI) for(n=1, 10^3, if(isprime(sigma(n)-1), print1(n, ", "))) \\ Derek Orr, Nov 01 2014

CROSSREFS

Cf. A000203 (sum of divisors), A000040 (primes).

Cf. A039653 (sigma(n)-1), A066073 (subsequence of composites), A248793.

Cf. A065512 (numbers n such that sigma(n) + 1 is prime).

Sequence in context: A048839 A122144 A064052 * A064594 A325511 A240370

Adjacent sequences:  A248789 A248790 A248791 * A248793 A248794 A248795

KEYWORD

nonn,easy

AUTHOR

Jaroslav Krizek, Nov 01 2014

STATUS

approved

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Last modified October 14 12:00 EDT 2019. Contains 328003 sequences. (Running on oeis4.)