OFFSET
1,3
COMMENTS
Numbers n such that (360*10^n - 27)/9 is prime.
Numbers n such that digit 3 followed by n >= 0 occurrences of digit 9 followed by digit 7 is prime.
Numbers corresponding to terms <= 871 are certified primes.
a(14) > 10^5. - Robert Price, Mar 17 2015.
a(15) > 2*10^5. - Robert Price, Oct 02 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
LINKS
FORMULA
a(n) = A101398(n) - 1.
EXAMPLE
397 is prime, hence 1 is a term.
MATHEMATICA
Select[Range[0, 1000], PrimeQ[(360*10^# - 27)/9] &] (* Robert Price, Mar 17 2015 *)
PROG
(PARI) a=37; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a+27)
(PARI) for(n=0, 1500, if(isprime((360*10^n-27)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 20 2004
EXTENSIONS
8845 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
a(11)-a(13) derived from A101398 by Robert Price, Mar 17 2015
a(14) from Robert Price, Oct 02 2015
STATUS
approved