A124877 - OEIS

%I #16 Nov 16 2024 16:32:04

%S 3,15,19,26,31,129,139,211,242,246,251,474,552,558,694,801,1001,1123,

%T 1313,1687,4168,4484,5611,6869,12197,13472

%N Numbers k such that A007408(k) is prime.

%e a(1) = 3 since A007408(3)= 251 is the first prime number of sequence A007408,

%e a(2) = 15 since A007408(15) = 56154295334575853 is the second prime number of sequence A007408.

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

%o (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

%o (Python) # uses A007408gen() and imports from A007408

%o from sympy import isprime

%o def agen(): yield from (k for k, ak in enumerate(A007408gen(), 1) if isprime(ak))

%o print(list(islice(agen(), 15))) # _Michael S. Branicky_, Jun 26 2022

%Y Cf. A007408.

%Y Cf. A125503. [From _Alexander Adamchuk_, Apr 26 2010]

%K hard,more,nonn

%O 1,1

%A _M. F. Hasler_, Nov 11 2006

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

%E a(21)-a(24) from _Alexander Adamchuk_, Apr 26 2010

%E a(25)-a(26) from _Michael S. Branicky_, Nov 16 2024