OFFSET
1,3
COMMENTS
Numbers n such that (240*10^n + 21)/9 is prime.
Numbers n such that digit 2 followed by n >= 0 occurrences of digit 6 followed by digit 9 is prime.
Numbers corresponding to terms <= 456 are certified primes.
a(22) > 10^5. - Robert Price, Jan 29 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
LINKS
FORMULA
a(n) = A102957(n+1) - 1.
EXAMPLE
26669 is prime, hence 3 is a term.
PROG
(PARI) a=29; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a-21)
(PARI) for(n=0, 1500, if(isprime((240*10^n+21)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
_Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 23 2004
EXTENSIONS
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
a(16)-a(21) derived from A102957 by _Robert Price_, Jan 29 2015
STATUS
approved