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A124274
Nonprime numbers k that divide A124271(k) = Sum_{i=1..k} (prime(i)^k - 1) / (prime(i) - 1).
2
1, 9, 15, 121
OFFSET
1,2
COMMENTS
The next term if it exists is greater than 1000.
Note that a(1) = 1, a(2) = 3^2 and a(4) = 11^2 are perfect squares.
EXAMPLE
9 is a term because 9 divides A124271(9) = 108336139407.
MATHEMATICA
s={}; Do[If[!PrimeQ[k], If[Divisible[Sum[(Prime[i]^k-1)/(Prime[i]-1), {i, k}], k], AppendTo[s, k]]], {k, 10^3}]; s (* James C. McMahon, Dec 10 2024 *)
CROSSREFS
Cf. A124271, A124273 (primes p that divide A124271(p)).
Sequence in context: A136354 A177184 A098146 * A075134 A316744 A158789
KEYWORD
bref,hard,more,nonn
AUTHOR
Alexander Adamchuk, Oct 23 2006
STATUS
approved