OFFSET
1,2
COMMENTS
Numbers n such that 40*10^n + 9 is prime.
Numbers n such that digit 4 followed by n >= 0 occurrences of digit 0 followed by digit 9 is prime.
Numbers corresponding to terms <= 195 are certified primes.
a(18) > 2*10^5. - Robert Price, May 24 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
LINKS
Makoto Kamada, Prime numbers of the form 400...009.
FORMULA
a(n) = A101394(n+1) - 1.
EXAMPLE
40009 is prime, hence 3 is a term.
MATHEMATICA
Select[Range[0, 1000], PrimeQ[40*10^# + 9] &] (* Robert Price, May 24 2015 *)
PROG
(PARI) a=49; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a-81)
(PARI) for(n=0, 1500, if(isprime(40*10^n+9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 14 2004
EXTENSIONS
2037 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
a(10)-a(11) from Kamada data by Ray Chandler, Apr 30 2015
a(12)-a(17) from Robert Price, May 24 2015
STATUS
approved