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Old 2020-01-27, 14:30   #299
Dr Sardonicus
 
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Feb 2017
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3*2^5523861 + 1 is definitely composite. From the List of primes k*2n + 1 for k < 300 we have for k = 3,
Quote:
3 1, 2, 5, 6, 8, 12, 18, 30, 36, 41, 66, 189, 201, 209, 276, 353, 408, 438, 534, 2208, 2816, 3168, 3189, 3912, 20909, 34350, 42294, 42665, 44685, 48150, 54792, 55182, 59973, 80190, 157169, 213321, 303093, 362765, 382449, 709968, 801978, 916773, 1832496, 2145353, 2291610, 2478785, 5082306, 7033641, 10829346 [14884000]
(The exponents in bold give factors of Fermat numbers.)
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