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A343120
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a(n) is the smallest number b such that (b^(2^k) + 1)/2 is prime for k = 0, 1, ..., n.
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1
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OFFSET
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0,1
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LINKS
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EXAMPLE
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For n=3, the four numbers (205+1)/2, (205^2+1)/2, (205^4+1)/2, and (205^8+1)/2 are prime, and 205 is smallest with this property, so a(3)=205.
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PROG
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(PARI) a(n)=forstep(b=3, +oo, 2, for(k=0, n, !ispseudoprime((b^(2^k)+1)/2)&&next(2)); return(b)) \\ if a(n-1) is known, b loop can start from there instead
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CROSSREFS
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KEYWORD
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nonn,more,hard
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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