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

1,1

Links

Table of n, a(n) for n=1..26.

Example

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.

Maple

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

Prog

(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

Crossrefs

Cf. A007408.

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

Sequence in context: A206367 A039570 A032644 * A111517 A083545 A224872

Adjacent sequences: A124874 A124875 A124876 * A124878 A124879 A124880

Keyword

hard,more,nonn

Author

M. F. Hasler, Nov 11 2006

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

Status

approved