OFFSET
1,3
COMMENTS
Numbers n such that (180*10^n - 27)/9 is prime.
Numbers n such that digit 1 followed by n >= 0 occurrences of digit 9 followed by digit 7 is prime.
Numbers corresponding to terms <= 987 are certified primes.
a(21) > 10^5. - Robert Price, Nov 16 2014
a(24) > 2*10^5. - Robert Price, Jul 11 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
LINKS
FORMULA
a(n) = A102947(n) - 1. - Robert Price, Nov 16 2014
EXAMPLE
197 is prime, hence 1 is a term.
MATHEMATICA
Select[Range[0, 1000], PrimeQ[(180 10^# - 27) / 9] &] (* Vincenzo Librandi, Nov 17 2014 *)
PROG
(PARI) a=17; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+27)
(PARI) for(n=0, 1500, if(isprime((180*10^n-27)/9), print1(n, ", ")))
(Magma) [n: n in [0..500] | IsPrime((180*10^n-27) div 9) ]; // Vincenzo Librandi, Nov 17 2014
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 28 2004
EXTENSIONS
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
a(18)-a(20) derived from A102947 by Robert Price, Nov 16 2014
a(21)-a(23) from Robert Price, Jul 11 2015
STATUS
approved