A068368 Composite numbers k such that sigma(2*k+1)-sigma(k) = k+1.

16, 335, 8399, 12599, 16383, 62999, 546359, 743999, 2815199, 3046655, 3592655, 8422679, 25357439, 26356199, 29318279, 75428999, 112039199, 202417343, 226325999, 329611559, 335894159, 452921039, 589783295, 691547999, 696032399, 772098599, 1189437239, 1512741267

Offset

1,1

Comments

Primes satisfying sigma(2p+1)-sigma(p) = p+1 are Sophie Germain primes (A005384).

Links

Table of n, a(n) for n=1..28.

Mathematica

Select[Range[10^6], CompositeQ[#] && Subtract @@ DivisorSigma[1, {2*# + 1, #}] == # + 1 &] (* Amiram Eldar, Apr 20 2025 *)

Prog

(PARI) for(n=1, 500000000, if((sigma(2*n+1)-sigma(n)==n+1) && (!isprime(n)), print1(n, ", ")))

Crossrefs

Cf. A000203, A005384, A008438.

Sequence in context: A223394 A218476 A240344 * A013993 A058722 A318013

Adjacent sequences: A068365 A068366 A068367 * A068369 A068370 A068371

Keyword

nonn

Author

Benoit Cloitre, Feb 28 2002

Extensions

More terms from Rick L. Shepherd, May 08 2002

a(23)-a(28) from Amiram Eldar, Apr 20 2025

Status

approved