OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digits 15 followed by k-1 occurrences of the digit 0 followed by the digit 1 is prime (see Example section).
a(23) > 10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 150w1.
EXAMPLE
4 is in this sequence because 15*10^4 + 1 = 150001 is prime.
Initial terms and associated primes:
a(1) = 1, 151;
a(2) = 4, 150001;
a(3) = 7, 150000001;
a(4) = 8, 1500000001;
a(5) = 18, 15000000000000000001; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[15*10^# + 1] &]
PROG
(PARI) isok(k) = isprime(15*10^k + 1); \\ Michel Marcus, Nov 20 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 19 2017
STATUS
approved