OFFSET
1,3
COMMENTS
Numbers n such that 90*10^n + 7 is prime.
Numbers n such that digit 9 followed by n >= 0 occurrences of digit 0 followed by digit 7 is prime.
Numbers corresponding to terms <= 1349 are certified primes.
a(30) > 2*10^5. - Robert Price, Oct 16 2015
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
LINKS
FORMULA
a(n) = A096774(n) - 1.
EXAMPLE
90007 is prime, hence 3 is a term.
MATHEMATICA
Select[Range[0, 200000], PrimeQ[90*10^# + 7] &] (* Robert Price, Oct 16 2015 *)
PROG
(PARI) a=97; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a-63)
(PARI) for(n=0, 1500, if(isprime(90*10^n+7), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Nov 27 2004
EXTENSIONS
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(25)-a(29) from Kamada data by Ray Chandler, Apr 28 2015
STATUS
approved