A342630 -id:A342630 - cp's OEIS frontend

2, 2, 7, 23, 19, 43, 137, 139, 239, 283, 373, 491, 1019, 929, 2609, 2699, 1499, 4451, 3191, 4441, 5261, 3251, 6373, 7853, 8623, 9013, 6359, 10289, 9109, 6833, 7703, 13417, 19441, 15329, 19793, 9311, 16319, 13109, 12539, 23899, 26347, 10111, 11351, 19687, 27851, 13627, 34129, 18521, 27277, 35537

Offset: 1

J. M. Bergot and Robert Israel, Mar 17 2021

nonn

Terms are listed in the order in which they appear in A340180, including repetitions.

Repeated terms include a(1) = a(2) = 2, a(55) = a(56) = 17333, and a(233) = a(319) = 1253761. Are there any others?

			a(4) = A340180(A342630(4)) = A340180(14) = 23.

Robert Israel, Table of n, a(n) for n = 1..5000

Cf. A340180, A342630.

f:= proc(n) local s,k;
  s:= numtheory:-sigma(n);
  add(`if`(igcd(n,k)=1, s mod k, 0),k=1..n-1);
end proc:
select(isprime,map(f, [$1..1000]));

Select[Array[Sum[Mod[DivisorSigma[1, #], k] Floor[1/GCD[k, #]], {k, # - 1}] &, 500], PrimeQ] (* Michael De Vlieger, Mar 17 2021 *)

a(n) = A340180(A342630(n)).

cp's OEIS Frontend

A342644 Primes in A340180.

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