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
LINKS
Makoto Kamada, Search for 53w23.
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
CROSSREFS
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