A124877 Numbers k such that A007408(k) is prime.

3, 15, 19, 26, 31, 129, 139, 211, 242, 246, 251, 474, 552, 558, 694, 801, 1001, 1123, 1313, 1687, 4168, 4484, 5611, 6869, 12197, 13472

Offset: 1

Author

M. F. Hasler, Nov 11 2006

Keywords

• hard
• more
• nonn

Examples

			a(1) = 3 since A007408(3)= 251 is the first prime number of sequence A007408,
a(2) = 15 since A007408(15) = 56154295334575853 is the second prime number of sequence A007408.
		

Crossrefs

Cf. A007408.

Cf. A125503. [From Alexander Adamchuk, Apr 26 2010]

Programs

Maple

select(isprime@A007408,[$1..200]);

PARI

num=1;den=1;for(k=2,2000,k3=k^3;s=(num*k3+den)/(den*k3);num=numerator(s);den=denominator(s);if(isprime(num),print1(k","))) \\ Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 26 2008

Python

# uses A007408gen() and imports from A007408
from sympy import isprime
def agen(): yield from (k for k, ak in enumerate(A007408gen(), 1) if isprime(ak))
print(list(islice(agen(), 15))) # Michael S. Branicky, Jun 26 2022

Extensions

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 26 2008

a(21)-a(24) from Alexander Adamchuk, Apr 26 2010

a(25)-a(26) from Michael S. Branicky, Nov 16 2024