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A004249
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a(n) = (2^2^...^2) (with n 2's) + 1.
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21
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OFFSET
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0,1
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COMMENTS
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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
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REFERENCES
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P. Ribenboim, The Book of Prime Number Records. Springer-Verlag, NY, 2nd ed., 1989, p. 73.
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LINKS
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FORMULA
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a(0) = 2, a(n) = 2^a(n-1)/2 + 1 for n >= 1.
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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