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It appears that n k is in the sequence iff n k is prime or n k is in {1, 21, 822857} (Verified to 3.1*10^6). It is true that if n k is the product of two distinct primes, then nk=21. - George J. Schaeffer (gschaeff(AT)andrew.cmu.edu), Apr 30 2005, R. J. Mathar, Jan 25 2017
Numbers n k such that the remainder of the harmonic residue of n k when divided by n k is nk-1.
The are no other nonprime terms below 10^11. - Amiram Eldar, Jan 09 2024
(PARI) is(n) = {my(f = factor(n)); n*numdiv(f) % sigma(f) == n - 1; } \\ Amiram Eldar, Jan 09 2024
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editing
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It appears that n is in the sequence iff n is prime or n is in {1, 21, 822857} (Verified to 3.1*10^6). It is true that if n is the product of two distinct primes, then n=21. - George J. Schaeffer (gschaeff(AT)andrew.cmu.edu), Apr 30 2005, _R. J. Mathar_, Jan 25 2017
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editing
It appears that n is in the sequence iff n is prime or n is in {1, 21} (Verified to 10^6). It is true that if n is the product of two distinct primes, then n=21. - George J. Schaeffer (gschaeff(AT)andrew.cmu.edu), Apr 30 2005
nonn,new
nonn