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A309744
Numbers k such that 81*10^(2*k) + 9*10^k + 1 is prime.
1
1, 3, 17, 31, 7101, 14183
OFFSET
1,2
EXAMPLE
91 = 7 * 13.
8191 is prime ==> a(1) = 0.
810901 = 7^2 * 13 * 19 * 67.
81009001 is prime ==> a(2) = 3.
8100090001 = 73 * 271 * 331 * 1237.
810000900001 = 1471 * 550646431.
81000009000001 = 7 * 13 * 613 * 757 * 1129 * 1699.
8100000090000001 = 31 * 439 * 595194363289.
810000000900000001 = 7 * 13 * 157 * 193 * 4243 * 4969 * 13933.
81000000009000000001 = 271 * 298892988963099631.
PROG
(PARI) for(k=0, 1e3, if(ispseudoprime(81*100^k+9*10^k+1), print1(k", ")))
CROSSREFS
Sequence in context: A321796 A372797 A214093 * A115355 A146021 A146133
KEYWORD
nonn,more,base
AUTHOR
Seiichi Manyama, Aug 15 2019
EXTENSIONS
a(6) from Michael S. Branicky, Sep 05 2024
STATUS
approved