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A295400
Numbers k such that 3*10^k - 23 is prime.
0
1, 2, 5, 8, 14, 18, 20, 36, 68, 224, 252, 563, 780, 2430, 3150, 7919, 11092, 14020, 14908, 58032
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 9 followed by the digits 77 is prime (see Example section).
a(21) > 2*10^5.
EXAMPLE
2 is in this sequence because 3*10^2 - 23 = 277 is prime.
Initial terms and associated primes:
a(1) = 1, 7;
a(2) = 2, 277;
a(3) = 5, 299977;
a(4) = 8, 299999977;
a(5) = 14, 299999999999977; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[3*10^# - 23] &]
PROG
(PARI) isok(k) = isprime(3*10^k - 23); \\ Michel Marcus, Nov 22 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 21 2017
STATUS
approved