|
COMMENTS
|
Sequences of numbers m with n such divisors for 1 < n < 6:
n = 2: 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, ... (A065091 - odd primes).
n = 3: 49, 169, 361, 961, 1369, 1849, 3721, 4489, 5329, 6241, 9409, ...
n = 4: 15, 21, 33, 35, 39, 51, 55, 57, 65, 69, 77, 85, 87, 91, 93, ...
n = 5: 923521, 13845841, 519885601, 1073283121, 1982119441, ...
Other terms: a(18) = 24843, a(24) = 8085, a(32) = 15015.
a(p) = (k*p# + 1)^(p-1) for some k > 0 and p# is the product of primes <= k. - David A. Corneth, May 01 2020 [Proof: a(p) must be of the form q^(p-1), where q is a prime. Thus r | (q^r - 1)/(q - 1) if r <= p is prime. Suppose that q - 1 is not divisible by r, then 0 == q^r - 1 == q - 1 (mod r), a contradiction! Therefore, q - 1 is divisible by any primes r <= p. In conclusion, q = k*p# + 1 for some k > 0. - Jinyuan Wang, May 02 2020]
a(17) = 4084081^16 and a(19) = 106696591^18 are too large to be included. - Amiram Eldar, May 02 2020
|