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sigma(n) is the sum of the divisors of n (A000203).
Numbers n such that A000203(A000169(n)) is a prime.
a(8) > 10^4.
Corresponding values of n^(n-1): 2, 9, 64, 1152921504606846976, ...
Corresponding values of sigma(n^(n-1)): 3, 13, 127, 2305843009213693951, ...
Subsequence of A280257 (numbers n such that tau(n^(n-1)) is prime).
Prime terms are in A088790.
For n < 1000; sigma(n^(n+1)) is prime only for n = 5; sigma(5^6) = sigma(15625) = 19531 (prime).
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