OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 3 followed by the digits 57 is prime (see Example section).
a(41) > 2*10^5.
LINKS
Makoto Kamada, Search for 23w57.
EXAMPLE
3 is in this sequence because (7*10^3 + 71)/3 = 2357 is prime.
Initial terms and associated primes:
a(1) = 1, 47;
a(2) = 2, 257;
a(3) = 3, 2357;
a(4) = 4, 23357;
a(5) = 5, 233357, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(7*10^# + 71)/3] &]
PROG
(PARI) lista(nn) = {for(n=1, nn, if(ispseudoprime((7*10^n + 71)/3), print1(n, ", "))); } \\ Altug Alkan, Mar 23 2016
CROSSREFS
KEYWORD
nonn
AUTHOR
Robert Price, Mar 23 2016
EXTENSIONS
a(38)-a(40) from Robert Price, May 21 2018
STATUS
approved