OFFSET
1,2
COMMENTS
Numbers n such that (650*10^n + 61)/9 is prime.
Numbers n such that digit 7 followed by n >= 0 occurrences of digit 2 followed by digit 9 is prime.
Numbers corresponding to terms <= 296 are certified primes.
a(16) > 10^5. - Robert Price, Oct 01 2015
All a(n) == 0, 2 or 3 mod 6 (cf. A047244). - Robert Israel, Oct 01 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
LINKS
FORMULA
a(n) = A103054(n) - 1.
EXAMPLE
72222229 is prime, hence 6 is a term.
MAPLE
select(t -> isprime((650*10^t + 61)/9), [seq(seq(6*s+i, i=[0, 2, 3]), s=0..700)]); # Robert Israel, Oct 01 2015
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(650*10^# + 61)/9] &] (* Robert Price, Oct 01 2015 *)
PROG
(PARI) a=79; for(n=0, 1000, if(isprime(a), print1(n, ", ")); a=10*a-61)
(PARI) for(n=0, 1000, if(isprime((650*10^n+61)/9), print1(n, ", ")))
(Magma) [n: n in [0..3*10^2]| IsPrime((650*10^n+61) div 9)]; // Vincenzo Librandi, Oct 02 2015
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 03 2004
EXTENSIONS
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(15) from Erik Branger May 01 2013 by Ray Chandler, Apr 30 2015
STATUS
approved