OFFSET
1,2
COMMENTS
Numbers n such that (640*10^n + 17)/9 is prime.
Numbers n such that digit 7 followed by n >= 0 occurrences of digit 1 followed by digit 3 is prime.
Numbers corresponding to terms <= 937 are certified primes.
a(8) > 10^5. - Robert Price, Sep 18 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
LINKS
FORMULA
a(n) = A103050(n) - 1.
EXAMPLE
71111113 is prime, hence 6 is a term.
MATHEMATICA
Flatten[Position[NestList[10#-17&, 73, 1500], _?PrimeQ]]-1 (* Harvey P. Dale, Aug 15 2014 *)
PROG
(PARI) a=73; for(n=0, 1000, if(isprime(a), print1(n, ", ")); a=10*a-17)
(PARI) for(n=0, 1000, if(isprime((640*10^n+17)/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(5)-a(6) from Kamada data by Ray Chandler, Apr 29 2015
a(7) from Robert Price, Sep 18 2015
STATUS
approved