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Search: id:A120375
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| A120375 |
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Integers n such that 2*5^n-1 is prime. |
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+0
1
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| 4, 6, 16, 24, 30, 54, 96, 178, 274, 1332, 2766, 3060, 4204, 17736
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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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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FORMULA
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a(n) = n-th number k such that 2*5^k-1 that is prime.
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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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CROSSREFS
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Cf. A000043, A000668, A002957, A002959, A003307, A079363, A055558.
Adjacent sequences: A120372 A120373 A120374 this_sequence A120376 A120377 A120378
Sequence in context: A132344 A097970 A044858 this_sequence A025618 A133572 A121852
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KEYWORD
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more,nonn
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AUTHOR
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Walter Kehowski (wkehowski(AT)cox.net), Jun 28 2006
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EXTENSIONS
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More terms from Ryan Propper (rpropper(AT)stanford.edu), Mar 28 2007
17736 from Herman Jamke (hermanjamke(AT)fastmail.fm), May 02 2007
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