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A270929
Numbers k such that (16*10^k - 31)/3 is prime.
496
1, 2, 3, 4, 15, 20, 24, 32, 38, 40, 63, 93, 104, 194, 208, 514, 535, 600, 928, 1300, 1485, 1780, 2058, 3060, 3356, 3721, 6662, 11552, 15482, 23000, 27375, 34748, 57219, 61251, 85221, 99656, 103214, 103244, 276537
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 3 followed by the digits 23 is prime (see Example section).
a(40) > 3*10^5. - Robert Price, Jul 13 2023
EXAMPLE
3 is in this sequence because (16*10^3 - 31)/3 = 5323 is prime.
Initial terms and associated primes:
a(1) = 1, 43;
a(2) = 2, 523;
a(3) = 3, 5323;
a(4) = 4, 53323;
a(5) = 15, 5333333333333323;
a(6) = 20, 533333333333333333323, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(16*10^# - 31)/3] &]
PROG
(PARI) isok(n) = ispseudoprime((16*10^n - 31)/3); \\ Michel Marcus, Mar 26 2016
KEYWORD
nonn,base,more
AUTHOR
Robert Price, Mar 26 2016
EXTENSIONS
a(37)-a(38) from Robert Price, Mar 03 2019
a(39) from Robert Price, Jul 13 2023
STATUS
approved