OFFSET
1,2
COMMENTS
Numbers n such that (350*10^n - 71)/9 is prime.
Numbers n such that digit 3 followed by n >= 0 occurrences of digit 8 followed by digit 1 is prime.
Numbers corresponding to terms <= 537 are certified primes.
a(23) > 10^5. - Robert Price, May 20 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
LINKS
FORMULA
a(n) = A102978(n) - 1.
EXAMPLE
3881 is prime, hence 2 is a term.
MATHEMATICA
#-1&/@(Flatten[Position[NestList[10#+71&, 31, 6200], _?(PrimeQ[#]&)]]) (* Harvey P. Dale, Aug 20 2011 *)
PROG
(PARI) a=31; for(n=0, 1700, if(isprime(a), print1(n, ", ")); a=10*a+71)
(PARI) for(n=0, 1700, if(isprime((350*10^n-71)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 20 2004
EXTENSIONS
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
a(17)-a(20) from Kamada data by Ray Chandler, May 01 2015
a(21)-a(22) from Robert Price, May 20 2015
STATUS
approved