A249902 Numbers n such that 2n-1 and sigma(n) are both primes.
2, 4, 9, 16, 64, 289, 1681, 2401, 3481, 4096, 15625, 65536, 85849, 262144, 491401, 531441, 552049, 683929, 703921, 734449, 1352569, 1885129, 3411409, 3892729, 5470921, 7091569, 7778521, 9247681, 10374841, 12652249, 18139081, 19439281, 22287841, 23902321
Offset: 1
Keywords
Examples
289 is in the sequence because 2*289 - 1 = 577 and sigma(289) = 307 (both primes).
Links
- Chai Wah Wu, Table of n, a(n) for n = 1..10622
Programs
-
Magma
[n: n in [2..10000000] | IsPrime(2*n-1) and IsPrime(SumOfDivisors(n))];
-
Mathematica
Select[Range[10^7], PrimeQ[2 # - 1] && PrimeQ[DivisorSigma[1, #]] &] (* Vincenzo Librandi, Nov 15 2014 *)
-
PARI
for(n=1,10^6,if(isprime(2*n-1)&&isprime(sigma(n)),print1(n,", "))) \\ Derek Orr, Nov 14 2014
-
Python
from sympy import isprime, divisor_sigma A249902_list = [2]+[n for n in (d**2 for d in range(1,10**3)) if isprime(2*n-1) and isprime(divisor_sigma(n))] # Chai Wah Wu, Jul 23 2016
Comments