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 A333973 #76 Oct 26 2020 23:01:11 %S A333973 18,20,21,54,147,342,602,889,258121 %N A333973 Numbers k such that A019320(k) is greater than A064078(k) and the latter is a prime or a prime power. %C A333973 The unique prime factor of A064078(k) is then a unique prime to base 2 (see A161509), but not a cyclotomic number. %C A333973 Subsequence of A161508. In fact, subsequence of the set difference A161508 \ A072226. %C A333973 In all known examples, A064078(k) is a prime. If A064078(k) was a prime power p^j with j>1, then p would be both a Wieferich prime (A001220) and a unique prime to base 2. %C A333973 Subsequence of A093106 (the characterization of A093106 can be useful when searching for more terms). %C A333973 Should this sequence be infinite? %H A333973 Henri Lifchitz and Renaud Lifchitz, <a href="http://www.primenumbers.net/prptop/searchform.php?form=Phi%28258121%2C2%29%2F719">Phi(258121,2)/719</a>. %H A333973 Wikipedia, <a href="https://en.wikipedia.org/wiki/Unique_prime#Binary_unique_primes">Unique prime, section Binary unique primes</a>. %o A333973 (PARI) for(n=1,+oo,c=polcyclo(n,2); c % n < 2 && next(); c/=(c%n); ispseudoprime(if(ispower(c,,&b),b,c))&&print1(n, ", ")) %Y A333973 Cf. A019320, A064078, A093106, A072226, A144755, A161508, A161509, A247071. %K A333973 nonn,hard,more %O A333973 1,1 %A A333973 _Jeppe Stig Nielsen_, Sep 22 2020