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 A004249 a(n) = (2^2^...^2) (with n 2's) + 1. 8
 2, 3, 5, 17, 65537 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS a(0) could equally well be taken to be 1 rather than 2, which gives A007516. - N. J. A. Sloane, Sep 14 2009 A subsequence of the Fermat numbers 2^2^n + 1 = A000215. a(0) through a(4) are primes; a(5) = 2^65536 + 1 is divisible by 825753601. a(5) = 20035299...19156737 has 19729 decimal digits. - Alois P. Heinz, Jun 15 2022 It is unknown if a(6) = A000215(65536) is composite. - Jeppe Stig Nielsen, Jun 15 2022 REFERENCES P. Ribenboim, The Book of Prime Number Records. Springer-Verlag, NY, 2nd ed., 1989, p. 73. LINKS Y. Bugeaud and M. Queffélec, On Rational Approximation of the Binary Thue-Morse-Mahler Number, Journal of Integer Sequences, 16 (2013), #13.2.3. Wilfrid Keller, Prime factors k.2^n + 1 of Fermat numbers F_m FORMULA a(0) = 2, a(n) = 2^a(n-1)/2 + 1 for n >= 1. a(n) = A014221(n) + 1. - Leroy Quet, Jun 10 2009, updated by Jeppe Stig Nielsen, Jun 15 2022 CROSSREFS Cf. Fermat numbers 2^2^n + 1 = A000215. A007516 is another version. Sequence in context: A127063 A127837 A253646 * A268210 A121510 A132346 Adjacent sequences:  A004246 A004247 A004248 * A004250 A004251 A004252 KEYWORD nonn AUTHOR STATUS approved

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Last modified October 6 12:35 EDT 2022. Contains 357264 sequences. (Running on oeis4.)