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A309740
Numbers k such that 5*10^(2*k) + 5*10^k + 1 are prime.
1
0, 3, 5, 301
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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 24 00:30 EDT 2024. Contains 371917 sequences. (Running on oeis4.)