A175727 Numbers k such that both A000203(k)-k-1 and A000203(k)-2*k-1 are prime numbers.

30, 42, 60, 70, 78, 102, 138, 186, 198, 216, 222, 228, 240, 246, 258, 270, 282, 360, 372, 390, 414, 438, 492, 498, 546, 582, 600, 606, 642, 708, 720, 756, 762, 774, 786, 810, 852, 870, 930, 942, 954, 1002, 1014, 1020, 1026, 1038, 1068, 1086, 1182, 1266, 1290

Offset: 1

Author

Max Alekseyev, Aug 18 2010

Keywords

• nonn

Comments

A simpler version of A179945, added at the request of the editor-in-chief.

Crossrefs

Subsequence of A179945.

Cf. A000203.

Programs

Maple

q:= n-> (t-> andmap(isprime, [t-n-1, t-2*n-1]))(numtheory[sigma](n)):
select(q, [$1..1500])[];  # Alois P. Heinz, Mar 21 2023

PARI

is(n)=my(s=sigma(n)); isprime(s-n-1) && isprime(s-n-n-1) \\ Charles R Greathouse IV, Feb 19 2013

Extensions

Edited by N. J. A. Sloane, Aug 28 2010