This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A121824 #13 Oct 23 2024 03:13:15
%S A121824 17,353,198593
%N A121824 Primes of the form (3^n + 5^n)/2.
%C A121824 Corresponding n are 2^1, 2^2, 2^3. What are the following terms? Cf. A074606 3^n + 5^n.
%C A121824 Since x^n + y^n has x+y as a factor if n is odd, we can assume that n is a power of 2. Maple shows that up to n = 2^15, there are no more primes of the form (3^n + 5^n)/2. This raises the question: Is it true that x^n + (x+2)^n is irreducible over Q for n a power of 2? - _W. Edwin Clark_, Sep 10 2006
%C A121824 Next term, if it exists, is > (3^2500+5^2500)/2. - _Hugo Pfoertner_, Sep 10 2006
%C A121824 No more terms <= (3^(2^17)+5^(2^17))/2=(3^131072+5^131072)/2. Hence the next term, if it exists, is greater than 10^91616 (so is too large to include). - Herman Jamke (hermanjamke(AT)fastmail.fm), Dec 31 2007
%t A121824 Select[Table[(3^n + 5^n)/2,{n,100}],PrimeQ] (* _James C. McMahon_, Oct 22 2024 *)
%o A121824 (PARI) for(n=1,17, m=(3^(2^n)+5^(2^n))/2;if(isprime(m),print1(m","))) \\ Herman Jamke (hermanjamke(AT)fastmail.fm), Dec 31 2007
%Y A121824 Cf. A074606, A121710.
%K A121824 hard,nonn,bref
%O A121824 1,1
%A A121824 _Zak Seidov_, Aug 27 2006
%E A121824 Edited by _N. J. A. Sloane_, Jan 13 2008