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    Sierpiński Number of the First Kind

    Definition

    A Sierpiński number of the first kind is a number of the form S_n congruent n^n + 1. The first few are 2, 5, 28, 257, 3126, 46657, 823544, 16777217, ... (OEIS A014566). Sierpiński proved that if S_n is prime with n>=2, then n must be of the form n = 2^(2^k), making S_n a Fermat number F_m with m = k + 2^k. The first few m of this form are 1, 3, 6, 11, 20, 37, 70, ... (OEIS A006127). The numbers of digits in the number S_k is given by d_k = ⌈2^(k + 2^k) log_10 2⌉, where ⌈z⌉ is the ceiling function, so the numbers of digits in the first few candidates are 1, 3, 20, 617, 315653, 41373247568, ... (OEIS A089943).

    Associated person

    Wacław Sierpiński

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