A067891 Primes p such that sigma(p+1) = sigma(p-1).

367, 919, 30593, 95393, 117571, 124759, 147341, 197261, 334541, 344417, 463219, 732257, 755081, 931757, 982759, 1996759, 2401219, 2962697, 3013447, 4722941, 7892827, 13333097, 13358407, 17946259, 19828483, 19855471, 19904981

Offset: 1

Author

Benoit Cloitre, Mar 02 2002

Keywords

• easy
• nonn

Links

• Giovanni Resta, Table of n, a(n) for n = 1..769 (terms < 10^12)

Crossrefs

Cf. A067889 (analog with tau).

Programs

Mathematica

fQ[p_] := DivisorSigma[1, p - 1] == DivisorSigma[1, p + 1]; p = 2; lst = {}; While[p < 100000000, If[fQ@ p, AppendTo[lst, p]]; p = NextPrime@ p]; lst (* Vladimir Joseph Stephan Orlovsky, Dec 03 2009 and modified by Robert G. Wilson v, May 08 2016 *)

PARI

for(n=2,10^8,if(isprime(n) && sigma(n+1)==sigma(n-1),print1(n,","))) \\ It is more efficient to use forprime(...).

PARI

is_A067891(p)=sigma(p-1)==sigma(p+1)&&isprime(p) \\ M. F. Hasler, Jul 31 2015

Extensions

More terms from Rick L. Shepherd, Apr 19 2002