OFFSET
1,2
COMMENTS
Numbers n such that (850*10^n - 31)/9 is prime.
Numbers n such that digit 9 followed by n >= 0 occurrences of digit 4 followed by digit 1 is prime.
Some of the larger entries may only correspond to probable primes.
a(10) > 10^5. - Robert Price, Nov 07 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
LINKS
FORMULA
a(n) = A103097(n) - 1.
EXAMPLE
944444441 is prime, hence 7 is a term.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(850*10^# - 31)/9] &] (* Robert Price, Nov 07 2015 *)
PROG
(PARI) a=91; for(n=0, 3000, if(isprime(a), print1(n, ", ")); a=10*a+31)
(PARI) for(n=0, 3000, if(isprime((850*10^n-31)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more,less
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Nov 27 2004
EXTENSIONS
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(7)-a(8) from Kamada data by Ray Chandler, Apr 28 2015
a(9) from Robert Price, Nov 07 2015
STATUS
approved