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A279793
Numbers k such that 4*10^k - 63 is prime.
0
2, 4, 5, 8, 9, 15, 19, 25, 26, 137, 141, 257, 399, 508, 524, 607, 709, 953, 989, 2484, 3196, 4538, 6448, 35401, 36106, 51530, 79327, 109316, 171891, 184004
OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 9 followed by the digits 37 is prime (see Example section).
a(31) > 3*10^5.
EXAMPLE
4 is in this sequence because 4*10^4 - 63 = 39937 is prime.
Initial terms and associated primes:
a(1) = 2, 337;
a(2) = 4, 39937;
a(3) = 5, 399937;
a(4) = 8, 399999937;
a(5) = 9, 3999999937; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[4*10^# - 63] &]
PROG
(PARI) is(n)=ispseudoprime(4*10^n - 63) \\ Charles R Greathouse IV, Jun 13 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Dec 18 2016
EXTENSIONS
a(28)-a(30) from Robert Price, Oct 24 2018
STATUS
approved