%I #21 Feb 07 2022 14:01:52

%S 41124,230867,358267,37539572,148025049,235167249,242788284,

%T 1085464188,142772845653,202728626748

%N Composite numbers k such that (k-1) divides (A018804(k)-1).

%C For any prime p, (p-1) divides (A018804(p)-1) = 2(p-1).

%C Some larger terms: 62763888399737.

%H Pruthviraj et al., <a href="https://mathoverflow.net/q/393216">Are there infinitely many composite a such that Sum_{k=1..a} (k,a) == 1 (mod a-1)?</a>, MathOverflow, 2021.

%t f[p_, e_] := (e*(p - 1)/p + 1)*p^e; pil[n_] := Times @@ f @@@ FactorInteger[n]; Select[Range[2, 400000], CompositeQ[#] && Divisible[pil[#] - 1, # - 1] &] (* _Amiram Eldar_, May 19 2021 *)

%Y Cf. A018804.

%K nonn,hard,more

%O 1,1

%A _Max Alekseyev_, May 19 2021

%E a(9)-a(10) confirmed by _Martin Ehrenstein_, May 27 2021