OFFSET
1,2
COMMENTS
Numbers n such that (720*10^n - 81)/9 is prime.
Numbers n such that digit 7 followed by n >= 0 occurrences of digit 9 followed by digit 1 is prime.
Numbers corresponding to terms <= 761 are certified primes.
a(24) > 2*10^5. - Robert Price, Dec 19 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
LINKS
FORMULA
a(n) = A103068(n) - 1.
EXAMPLE
799991 is prime, hence 4 is a term.
MATHEMATICA
Select[Range[0, 300], PrimeQ[(720*10^# - 81)/9] &] (* Robert Price, Dec 19 2015 *)
PROG
(PARI) a=71; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+81)
(PARI) for(n=0, 1500, if(isprime((720*10^n-81)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 03 2004
EXTENSIONS
a(19) from Mohammed Bouayoun (Mohammed.Bouayoun(AT)sanef.com), Apr 13 2006
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(22)-a(23) from Kamada data by Ray Chandler, Apr 30 2015
STATUS
approved