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A309740 Numbers k such that 5*10^(2*k) + 5*10^k + 1 are prime. 1
0, 3, 5, 301 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

11 | 5*10^(4*m) + 5*10^(2*m) + 1. So a(n) is odd for n > 1.

LINKS

Table of n, a(n) for n=1..4.

EXAMPLE

           11 is prime ==> a(1) = 0.

          551 = 19 * 29.

        50501 = 11 * 4591.

      5005001 is prime ==> a(2) = 3.

    500050001 = 11 * 61 * 745231.

  50000500001 is prime ==> a(3) = 5.

5000005000001 = 11 * 31 * 1801 * 8141461.

PROG

(PARI) for(k=0, 1e3, if(ispseudoprime(5*100^k+5*10^k+1), print1(k", ")))

CROSSREFS

Cf. A309739.

Sequence in context: A101331 A232239 A087368 * A280035 A087670 A271390

Adjacent sequences:  A309737 A309738 A309739 * A309741 A309742 A309743

KEYWORD

nonn,base,more

AUTHOR

Seiichi Manyama, Aug 15 2019

STATUS

approved

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Last modified April 22 21:05 EDT 2021. Contains 343177 sequences. (Running on oeis4.)