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A120375
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Integers k such that 2*5^k - 1 is prime.
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5
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4, 6, 16, 24, 30, 54, 96, 178, 274, 1332, 2766, 3060, 4204, 17736, 190062, 223536, 260400, 683080
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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.
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LINKS
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FORMULA
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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[Range[0, 100], PrimeQ[2*5^# - 1] &] (* Robert Price, Mar 14 2015 *)
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PROG
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(PARI) isok(k) = ispseudoprime(2*5^k-1); \\ Altug Alkan, Sep 22 2018
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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a(14) from Herman Jamke (hermanjamke(AT)fastmail.fm), May 02 2007
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STATUS
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approved
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