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 A278021 #8 Nov 08 2016 15:03:04 %S A278021 4,9,15,25,33,35,49,65,69,77,87,91,95,115,119,121,123,143,159,169,185, %T A278021 187,215,221,247,249,255,259,267,287,289,295,299,319,323,329,339,341, %U A278021 361,365,377,393,395,407,413,415,427,437,455,473,485,511,515,519 %N A278021 Numbers n such that n - lambda(n) is prime, where lambda = A002322. %C A278021 All terms are composite. %C A278021 4 is the only even term. %C A278021 For odd primes p, 3*p is a term iff p is in A005384. %H A278021 Robert Israel, <a href="/A278021/b278021.txt">Table of n, a(n) for n = 1..10000</a> %H A278021 Wikipedia, <a href="https://en.wikipedia.org/wiki/Carmichael_function">Carmichael function</a> %F A278021 A010051(n - A002322(n)) = 1. %e A278021 25-lambda(25) = 25-20 = 5 is prime so 25 is in the sequence. %p A278021 select(t -> isprime(t - numtheory:-lambda(t)), [$1..10000]); %Y A278021 Cf. A002322, A005384, A010051, A277127. %Y A278021 Contains A001248. Contained in union of A001248 and A024556. %K A278021 nonn %O A278021 1,1 %A A278021 _Robert Israel_, Nov 08 2016