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Just showing those entries submitted by 'Meiburg': (Click here to show all) All Mersenne primes are of the from 2^k  1, and k must be prime. For 8191, k=13. The case of k=11 (the prime preceding 13) gives the smallest composite Mersenne number, with one of it's factors being 89. If you remove every other digit, you get 89 as well. [Meiburg]Suppose you listed all primes, with three digits or less, using the digits in this number (1,8, and 9), without limiting the number of times a number can use each digit (i.e., it may have two nines). If they are now listed from least to greatest with signs in the order "+, +, ,", beginning with 11+18+19181+191+199811... the sum is 1381, which is a prime as well. [Meiburg]
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