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A120376
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Primes of the form 2*5^k - 1.
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2
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1249, 31249, 305175781249, 119209289550781249, 1862645149230957031249, 111022302462515654042363166809082031249, 25243548967072377773175314089049159349542605923488736152648925781249
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OFFSET
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1,1
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COMMENTS
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See comments for A057472. Examined in base 12, all n must be even and all primes must be 1-primes. For example, 1249 is 881 in base 12.
The next term has 125 digits. - Harvey P. Dale, Jan 26 2019
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LINKS
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Table of n, a(n) for n=1..7.
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FORMULA
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a(n) = 2*5^A120375(n) - 1 = 2*5^(2*A002958(n)) - 1. - Jianing Song, Sep 22 2018
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EXAMPLE
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a(1) = 4 since 2*5^4 - 1 = 1249 is the first prime.
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MAPLE
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for w to 1 do for k from 1 to 2000 do n:=2*5^k-1; if isprime(n) then printf("%d, %d", k, n) fi od od;
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MATHEMATICA
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Select[2*5^Range[100]-1, PrimeQ] (* Harvey P. Dale, Jan 26 2019 *)
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PROG
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(PARI) for(k=1, 1e3, if(ispseudoprime(p=2*5^k-1), print1(p, ", "))); \\ Altug Alkan, Sep 22 2018
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CROSSREFS
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Integers k such that 2*b^k - 1 is prime: A090748 (b=2), A003307 (b=3), A120375 (b=5), A057472 (b=6), A002959 (b=7), A002957 (b=10), A120378 (b=11).
Primes of the form 2*b^k - 1: A000668 (b=2), A079363 (b=3), this sequence (b=5), A158795 (b=7), A055558 (b=10), A120377 (b=11).
Cf. also A000043, A002958.
Sequence in context: A020384 A086709 A215719 * A231805 A122272 A330650
Adjacent sequences: A120373 A120374 A120375 * A120377 A120378 A120379
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KEYWORD
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nonn
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AUTHOR
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Walter Kehowski, Jun 28 2006
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STATUS
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approved
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