OFFSET
1,1
COMMENTS
For any prime p, (p-1) divides (A018804(p)-1) = 2(p-1).
Some larger terms: 62763888399737.
LINKS
Pruthviraj et al., Are there infinitely many composite a such that Sum_{k=1..a} (k,a) == 1 (mod a-1)?, MathOverflow, 2021.
MATHEMATICA
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 *)
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Max Alekseyev, May 19 2021
EXTENSIONS
a(9)-a(10) confirmed by Martin Ehrenstein, May 27 2021
STATUS
approved