OFFSET
1,1
COMMENTS
Numbers n such that (770*10^n - 41)/9 is prime.
Numbers n such that digit 8 followed by n >= 0 occurrences of digit 5 followed by digit 1 is prime.
Numbers corresponding to terms <= 470 are certified primes.
a(7) > 10^5. - Robert Price, Oct 21 2015
It appears that a(n)+1 are all divisible by 3. - Robert Price, Oct 21 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
LINKS
FORMULA
a(n) = A103083(n) - 1. - Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
EXAMPLE
8555555555555555555551 is prime, hence 20 is a term.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(770*10^# - 41)/9] &] (* Robert Price, Oct 21 2015 *)
Flatten[Position[NestList[10#+41&, 81, 5800], _?(PrimeQ[#]&)]]-1 (* Harvey P. Dale, May 09 2018 *)
PROG
(PARI) a=81; for(n=0, 1200, if(isprime(a), print1(n, ", ")); a=10*a+41)
(PARI) for(n=0, 1200, if(isprime((770*10^n-41)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Nov 30 2004
EXTENSIONS
One more term from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
STATUS
approved