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 2018-01-03, 23:26 #9 JeppeSN     "Jeppe" Jan 2016 Denmark 23×3×5 Posts This is related to one way you can generalize Mersenne primes $$2^n - 1$$. If you try to change the $$2$$ into $$A+1$$ with $$A>1$$, we have seen above that the resulting $$(A+1)^n-1$$ can never be prime because it is divisible by $$A$$. Therefore we can consider instead $\frac{(A+1)^n-1}{A}$ which may or may not be prime, just like Mersenne numbers. This is what some people call repunits in base $$A+1$$. For example $$\frac{10^{19}-1}{9}$$ or 1111111111111111111 is prime. /JeppeSN Last fiddled with by JeppeSN on 2018-01-03 at 23:27