Did you mean what Mersenne numbers are prime? If a number is a prime, how is it not a prime at the same time? Anyways, M11, or (2^11) - 1, I think is the lowest Mersenne Number Mp that isn't prime, when p is prime. Any Mersenne Number where p is not prime cannot be prime.
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Let p = any prime number. (2p -1) is called a Mersenne number. Any such number that is prime is called a Mersenne Prime. Father Mersenne wrote a list of numbers of this type which he thought were prime, but a few were not. In fact, most of the large Mersenne numbers are not prime, but all the really large numbers that have been proved to be prime are Mersenne Primes.
The algorithm for a Mersenne prime is 2n - 1, where n is a prime number and the solution is also a prime number.Applying the Mersenne algorithm to the prime numbers 2, 3, 5, and 7:22 - 1 = 3, 23 - 1 = 7, 25 - 1 = 31, and 27 - 1 = 127.3, 7, 32, and 127 all being prime numbers, this demonstrates the 2, 3, 5, and 7 are all Mersenne primes.
Their are many ways one that some times works is test those answers if prime are Mersenne primes. Or you could google"Is (put the questionable number here)prime?"
A Mersenne Prime is when 2^n-1 is prime. Some examples of a Mersenne Prime are n=2,3,5,7,13,17,19,31,61,89,107, and 127The first forty Mersenne primes are 2, 3, 5, 7, 13, 17, 19, 31, 61, 89, 107, 127, 521, 607, 1279, 2203, 2281, 3217, 4253, 4423, 9689, 9941, 11213, 19937, 21701, 23209, 44497, 86243, 110503, 132049, 216091, 756839, 859433, 1257787, 1398269, 2976221, 3021377, 6972593, 13466917, and 20996011.
1992 is not a Mersenne prime. 1992 is a composite number, being a multiple of 2.