OFFSET
1,1
COMMENTS
Numbers n such that (890*10^n - 71)/9 is prime.
Numbers n such that digit 9 followed by n >= 0 occurrences of digit 8 followed by digit 1 is prime.
Numbers corresponding to terms <= 413 are certified primes.
Certified primality of terms corresponding to 1191 and 1259 with Primo. - Ryan Propper, Jun 20 2005
a(14) > 10^5. - Robert Price, Nov 12 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
LINKS
FORMULA
a(n) = A103107(n+1) - 1.
EXAMPLE
9888881 is prime, hence 5 is a term.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(890*10^# - 71)/9] &] (* Robert Price, Nov 12 2015 *)
PROG
(PARI) a=91; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+71)
(PARI) for(n=0, 1500, if(isprime((890*10^n-71)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Nov 27 2004
EXTENSIONS
Two additional terms, corresponding to probable primes, from Ryan Propper, Jun 20 2005
Edited by T. D. Noe, Oct 30 2008
a(12) from Kamada data by Ray Chandler, Apr 29 2015
a(13) from Robert Price, Nov 12 2015
STATUS
approved