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Old 2020-09-29, 12:12   #4
Dr Sardonicus
 
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Feb 2017
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If x is a positive integer and f(x) = x*(x+1)^3 - 1 is a prime p, then sigma(p) = p+1 = x*(x+1)^3.

Everyone knows there are infinitely many x for which f(x) is prime (Bunyakovsky and Bateman-Horn conjectures), but nobody knows how to prove it.

The smallest such x is 1.
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