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 A381725 #10 Mar 06 2025 01:53:32 %S A381725 2,5,7,11,13,19,31,41,79,83,113,157,173,233,281,317,359,373 %N A381725 Numbers k such that 5^k - 4^k is a semiprime. %C A381725 Are all terms prime? Any term that is not a prime must be the square of a member of A059802. %H A381725 <a href="https://factordb.com/index.php?query=5%5E449-4%5E449">Status of 5^449-4^449 in factordb.com</a>. %F A381725 A001222(A005060(a(n))) = 2. %e A381725 a(3) = 7 is a term because 5^7 - 4^7 = 61741 = 29 * 2129 with 29 and 2129 prime. %p A381725 filter:= n -> numtheory:-bigomega(5^n - 4^n): %p A381725 select(filter, [2,seq(i,i=3..120,2]); %Y A381725 Cf. A001222, A001358, A005060, A059802, A082869. %K A381725 nonn,hard,more %O A381725 1,1 %A A381725 _Zak Seidov_ and _Robert Israel_, Mar 05 2025 %E A381725 a(14)-a(18) from _Jinyuan Wang_, Mar 05 2025