cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

A249763 Numbers k such that sigma(k) + 2 is prime.

Original entry on oeis.org

1, 2, 8, 18, 49, 72, 98, 128, 196, 200, 242, 288, 361, 392, 441, 676, 968, 1369, 1521, 1568, 1764, 1800, 2178, 2401, 2450, 2888, 3200, 3362, 3528, 3969, 4232, 5000, 5476, 5776, 5929, 6241, 6272, 7056, 7200, 7442, 7688, 7938, 8281, 8450, 8649, 9025, 9522, 9604
Offset: 1

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Author

Jaroslav Krizek, Nov 13 2014

Keywords

Comments

Except for 2, subsequence of nonprime numbers (A018252).
All odd terms are squares. - Michel Marcus, Dec 09 2014
Each term is either a square or twice a square. - Robert Israel, Dec 09 2014

Examples

			8 is in the sequence because sigma(8)+2 = 15+2 = 17 (prime).
		

Crossrefs

Programs

  • Magma
    [n: n in[1..100000] | IsPrime(SumOfDivisors(n) + 2)]
    
  • Maple
    N:= 10^5: # to get all terms <= N
    select(n -> isprime(numtheory:-sigma(n)+2), {seq(i^2,i=1..floor(sqrt(N))), seq(2*i^2,i=1..floor(sqrt(N/2)))} );
    # if using Maple 11 or earlier, uncomment the next line
    # sort(convert(%,list)); # Robert Israel, Dec 09 2014
  • Mathematica
    Select[Range[10000], PrimeQ[DivisorSigma[1, #] + 2] &] (* Vincenzo Librandi, Nov 14 2014 *)
  • PARI
    for(n=1,10^5,if(isprime(sigma(n)+2),print1(n,", "))) \\ Derek Orr, Nov 13 2014