OFFSET
1,2
COMMENTS
Numbers n such that (320*10^n - 41)/9 is prime.
Numbers n such that digit 3 followed by n >= 0 occurrences of digit 5 followed by digit 1 is prime.
Numbers corresponding to terms <= 272 are certified primes.
a(16) > 10^5. - Robert Price, May 24 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
LINKS
FORMULA
a(n) = A102971(n) - 1.
EXAMPLE
3555555551 is prime, hence 8 is a term.
MATHEMATICA
#-1&/@Flatten[Position[NestList[10#+41&, 31, 280], _?PrimeQ]] (* Harvey P. Dale, Jul 17 2011 *)
Select[Range[0, 10000], PrimeQ[(320 10^# - 41)/9] &] (* Vincenzo Librandi, May 25 2015 *)
PROG
(PARI) a=31; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+41)
(PARI) for(n=0, 1500, if(isprime((320*10^n-41)/9), print1(n, ", ")))
(Magma) [n: n in [0..2000] | IsPrime((320*10^n-41) div 9)]; // Vincenzo Librandi, May 26 2015
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 20 2004
EXTENSIONS
2487 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
a(13) from Kamada data by Ray Chandler, May 01 2015
a(14)-a(15) from Robert Price, May 24 2015
STATUS
approved