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Numbers k such that (10^k*113 + 19)/3 is prime.
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%I #38 Jul 22 2021 23:24:52

%S 1,9,13,33,81,1113,1609,5697,13577,49949

%N Numbers k such that (10^k*113 + 19)/3 is prime.

%C For k > 2, numbers such that '37'||...'6'...||'73' ('6' concatenated k-2 times and prefixed by '37', and suffixed by '73') in decimal form is prime.

%e 13 is in the sequence because (10^13*113 + 19)/3 = 376666666666673 is prime.

%t ParallelTable[ If[ PrimeQ[ (10^n*113+19)/3], n, Nothing], {n, 2000}]

%o (PARI) isok(n) = isprime((10^n*113 + 19)/3); \\ _Michel Marcus_, Jul 21 2017

%Y Cf. A051919, A101724.

%K nonn,hard,more

%O 1,2

%A _Mikk Heidemaa_, May 31 2017