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# Probable primes

For example, ${\displaystyle {\frac {45^{34351}-1}{44}}}$, a number of almost 57000 decimal digits, is a probable prime. A number like ${\displaystyle 2^{2^{127}-1}-1}$, on the other hand, might be prime, but it could just as easily be composite like the majority of small Mersenne numbers, and is thus not considered a probable prime.