A246858 - OEIS

%I #15 Sep 08 2022 08:46:09

%S 329,413,623,869,979,1819,2585,3107,3173,3197,3887,4235,4997,5771,

%T 6149,6187,6443,7409,8399,8759,14429,15323,18515,19019,21181,21413,

%U 23989,26491,29749,30355,31043,32623,34009,34177,39737,47321,47845,51389,53311,56419

%N Composite numbers k such that sigma(k + sigma(k)) = 2*sigma(k).

%C Complement of A005384 (Sophie Germain primes) with respect to A246857.

%e Number 329 (with sigma(329) = 384) is in sequence because sigma(329 + sigma(329)) = sigma(713) = 768 = 2*384.

%t Select[Range[57000], And[CompositeQ[#], DivisorSigma[1, # + DivisorSigma[1, #]] == 2 DivisorSigma[1, #]] &] (* _Michael De Vlieger_, Aug 05 2021 *)

%o (Magma) [n:n in[1..1000] | SumOfDivisors(n+SumOfDivisors(n)) eq 2*SumOfDivisors(n) and not IsPrime(n)]

%o (PARI) lista(nn) = {forcomposite(n=2, nn, if (sigma(n+sigma(n)) == 2*sigma(n), print1(n, ", ")););} \\ _Michel Marcus_, Sep 05 2014

%Y Cf. A074400, A246456.

%Y Cf. A005384, A246858.

%K nonn

%O 1,1

%A _Jaroslav Krizek_, Sep 05 2014