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A292201
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a(n) is the smallest value c such that prime(n)^c - 2 is prime, where prime(n) is the n-th prime or -1 if no such c exists.
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1
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2, 2, 1, 1, 4, 1, 6, 1, 24, 2, 1, 2, 4, 1, 2, 4, 4, 1, 3, 2, 1, 38, 4, 2, 747, 4, 1, 2, 1, 10, 2, 2, 10, 1, 50, 1, 22, 38, 12, 2, 40, 1, 2, 1, 164, 1, 2, 2, 12, 1, 2, 2, 1, 8, 2, 18, 22, 1, 3, 10, 1, 2, 102, 4, 1, 13896, 12, 2, 1122, 1
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OFFSET
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1,1
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COMMENTS
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a(71) > 38000 (if it exists). - Robert Price, Oct 23 2017
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LINKS
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Table of n, a(n) for n=1..70.
Carlos Rivera, Puzzle 887. p(n)^c-2 is prime, The Prime Puzzles and Problems Connection.
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EXAMPLE
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a(1) = 2 because 2^2 - 2 = 2 is prime;
a(2) = 2 because 3^2 - 2 = 7 is prime;
a(3) = 1 because 5^1 - 2 = 3 is prime;
a(4) = 1 because 7^1 - 2 = 5 is prime.
And these are the least exponents to satisfy the requested property.
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MATHEMATICA
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Table[c = 1; While[! PrimeQ[Prime[n]^c - 2], c++]; c, {n, 24}] (* Michael De Vlieger, Sep 11 2017 *)
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PROG
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(PARI) a(n) = {my(c = 1, p = prime(n)); while(!isprime(p^c-2), c++); c; }
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CROSSREFS
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Subsequence of A255707.
Sequence in context: A165509 A100996 A232504 * A343070 A090048 A064285
Adjacent sequences: A292198 A292199 A292200 * A292202 A292203 A292204
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KEYWORD
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nonn
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AUTHOR
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Michel Marcus, Sep 11 2017
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EXTENSIONS
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a(66)-a(70) from Robert Price, Oct 23 2017
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STATUS
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approved
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