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A123675
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a(n) = number of primes of the form 2^n - 5^k.
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0
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0, 1, 2, 1, 2, 1, 3, 2, 1, 1, 1, 2, 2, 1, 1, 0, 2, 2, 1, 1, 1, 1, 0, 2, 1, 4, 2, 3, 1, 0, 2, 2, 4, 1, 0, 3, 2, 2, 3, 0, 0, 2, 0, 3, 0, 1, 0, 1, 1, 2, 0, 3, 1, 0, 1, 2, 1, 1, 3, 0, 3, 2, 2, 2, 0, 3, 0, 2, 1, 0, 2, 1, 2, 2, 1, 2, 1, 1, 1, 1, 2, 1, 0, 0, 0, 0, 0, 2, 1, 1, 1, 2, 1, 3, 1, 0, 2, 2, 4, 2, 2, 1, 0, 3, 0
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,3
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COMMENTS
| a(1) = 0 because there are no prime numbers of the form 2^1 - 5^k. a(2) = 1 because the only prime of the form 2^2 - 5^k is 2^2 - 5^0 = 3. a(3) = 2 because there are two primes of the form 2^3 - 5^k: 2^3 - 3^0 = 7 and 2^3 - 5^1 = 3.
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MATHEMATICA
| Table[Length[Select[Range[0, Floor[Log[5, 2^n]]], PrimeQ[2^n-5^# ]&]], {n, 1, 150}]
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CROSSREFS
| Sequence in context: A134780 A154819 A104145 * A123400 A196059 A023116
Adjacent sequences: A123672 A123673 A123674 * A123676 A123677 A123678
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KEYWORD
| nonn
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AUTHOR
| Alexander Adamchuk (alex(AT)kolmogorov.com), Nov 17 2006
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