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A107123
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Numbers n such that (10^(2n+1)+18*10^n-1)/9 is prime.
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45
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OFFSET
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1,3
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COMMENTS
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A number n is in the sequence iff the palindromic number 1(n).3.1(n) is prime (1(n) means n copies of 1; dot between numbers means concatenation). If n is a positive term of the sequence then n is not of the form 3m, 6m+4, 12m+10, 28m+5, 28m+8, etc. (the proof is easy).
The palindromic number 1(n).2.1(n) is never prime for n > 0 because it is (1.0(n-1).1)*(1(n+1)). - Robert Israel, Jun 11 2015
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REFERENCES
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C. Caldwell and H. Dubner, "Journal of Recreational Mathematics", Volume 28, No. 1, 1996-97, pp. 1-9.
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LINKS
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FORMULA
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EXAMPLE
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19 is in the sequence because the palindromic number (10^(2*19+1)+18*10^19-1)/9 = 1(19).3.1(19) = 111111111111111111131111111111111111111 is prime.
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MAPLE
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select(n -> isprime((10^(2*n+1)+18*10^n-1)/9), [$0..100]); # Robert Israel, Jun 11 2015
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MATHEMATICA
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Do[If[PrimeQ[(10^(2n + 1) + 18*10^n - 1)/9], Print[n]], {n, 2500}]
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PROG
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(PARI) for(n=0, 1e4, if(ispseudoprime(t=(10^(2*n+1)+18*10^n)\9), print1(t", "))) \\ Charles R Greathouse IV, Jul 15 2011
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CROSSREFS
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KEYWORD
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nonn,base,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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