OFFSET
1,2
COMMENTS
Numbers n such that (260*10^n + 1)/9 is prime.
Numbers n such that digit 2 followed by n >= 0 occurrences of digit 8 followed by digit 9 is prime.
Numbers corresponding to terms <= 238 are certified primes.
a(18) > 2*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) = A102963(n+1) - 1.
EXAMPLE
28888889 is prime, hence 6 is a term.
MATHEMATICA
Flatten[Position[Table[FromDigits[Join[PadRight[{2}, n, 8], {9}]], {n, 1300}], _?PrimeQ]]-1 (* Harvey P. Dale, Sep 22 2012 *)
PROG
(PARI) a=29; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a-1)
(PARI) for(n=0, 1500, if(isprime((260*10^n+1)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 23 2004
EXTENSIONS
a(15)-a(17) derived from A102963 by Robert Price, Jan 29 2015
STATUS
approved