A278021 - OEIS

%I #8 Nov 08 2016 15:03:04

%S 4,9,15,25,33,35,49,65,69,77,87,91,95,115,119,121,123,143,159,169,185,

%T 187,215,221,247,249,255,259,267,287,289,295,299,319,323,329,339,341,

%U 361,365,377,393,395,407,413,415,427,437,455,473,485,511,515,519

%N Numbers n such that n - lambda(n) is prime, where lambda = A002322.

%C All terms are composite.

%C 4 is the only even term.

%C For odd primes p, 3*p is a term iff p is in A005384.

%H Robert Israel, <a href="/A278021/b278021.txt">Table of n, a(n) for n = 1..10000</a>

%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Carmichael_function">Carmichael function</a>

%F A010051(n - A002322(n)) = 1.

%e 25-lambda(25) = 25-20 = 5 is prime so 25 is in the sequence.

%p select(t -> isprime(t - numtheory:-lambda(t)), [$1..10000]);

%Y Cf. A002322, A005384, A010051, A277127.

%Y Contains A001248.  Contained in union of A001248 and A024556.

%K nonn

%O 1,1

%A _Robert Israel_, Nov 08 2016