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A143835
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a(n) = Number of x <= 10^n such that 2x^2-1 is prime.
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5
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OFFSET
| 1,1
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EXAMPLE
| a(1) = 7 because are 7 different x ={2, 3, 4, 6, 7, 8, 10} <= 10^1 where 2x^2-1 is prime = {7, 17, 31, 71, 97, 127, 199}
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MATHEMATICA
| l = 0; p = 2; a = {}; Do[k = p x^2 - 1; If[PrimeQ[k], l = l + 1]; If[N[Log[x]/Log[10]] == Round[N[Log[x]/Log[10]]], Print[l]; AppendTo[a, l]], {x, 1, 10000000}]; a (*Artur Jasinski*)
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CROSSREFS
| A066436, A066049, A090686, A090684, A143826, A143827, A143828, A143829, A143830, A143831, A143832, A143833, A143834
Sequence in context: A099842 A115194 A062274 * A103719 A134437 A018927
Adjacent sequences: A143832 A143833 A143834 * A143836 A143837 A143838
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KEYWORD
| base,nonn
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AUTHOR
| Artur Jasinski (grafix(AT)csl.pl), Sep 02 2008, Sep 04 2008
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