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A309740
Numbers k such that 5*10^(2*k) + 5*10^k + 1 is prime.
1
0, 3, 5, 301, 13817, 15259
OFFSET
1,2
COMMENTS
11 | 5*10^(4*m) + 5*10^(2*m) + 1. So a(n) is odd for n > 1.
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
KEYWORD
nonn,base,more
AUTHOR
Seiichi Manyama, Aug 15 2019
EXTENSIONS
a(5)-a(6) from Michael S. Branicky, Sep 03 2024
STATUS
approved