cp's OEIS Frontend

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

%I A124877 #16 Nov 16 2024 16:32:04
%S A124877 3,15,19,26,31,129,139,211,242,246,251,474,552,558,694,801,1001,1123,
%T A124877 1313,1687,4168,4484,5611,6869,12197,13472
%N A124877 Numbers k such that A007408(k) is prime.
%e A124877 a(1) = 3 since A007408(3)= 251 is the first prime number of sequence A007408,
%e A124877 a(2) = 15 since A007408(15) = 56154295334575853 is the second prime number of sequence A007408.
%p A124877 select(isprime@A007408,[$1..200]);
%o A124877 (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 A124877 (Python) # uses A007408gen() and imports from A007408
%o A124877 from sympy import isprime
%o A124877 def agen(): yield from (k for k, ak in enumerate(A007408gen(), 1) if isprime(ak))
%o A124877 print(list(islice(agen(), 15))) # _Michael S. Branicky_, Jun 26 2022
%Y A124877 Cf. A007408.
%Y A124877 Cf. A125503. [From _Alexander Adamchuk_, Apr 26 2010]
%K A124877 hard,more,nonn
%O A124877 1,1
%A A124877 _M. F. Hasler_, Nov 11 2006
%E A124877 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 26 2008
%E A124877 a(21)-a(24) from _Alexander Adamchuk_, Apr 26 2010
%E A124877 a(25)-a(26) from _Michael S. Branicky_, Nov 16 2024