A number satisfying Fermat's little theorem (or some other primality test) for some nontrivial base. A probable prime which is shown to be composite is called a pseudoprime (otherwise, of course, it is a prime). As of Sep. 2013, the largest known probable primes are the Wagstaff primes (2^13347311 + 1)/3 and (2^13372531 + 1)/3, both found by R. Propper in Sep. 2013 and which have 4017941 and 4025533 decimal digits, respectively. Other large known probable primes are the "dual Sierpinski numbers" 2^n + k given by 2^9092392 + 40291 and 2^5146295 + 41693, which have 2737083 and 1549190 decimal digits, respectively (Lifchitz and Lifchitz).

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