login
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
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.
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: A360477 A122144 A064052 * A358976 A064594 A325511
KEYWORD
nonn,easy
AUTHOR
Jaroslav Krizek, Nov 01 2014
STATUS
approved