OFFSET
1,3
COMMENTS
Numbers n such that 60*10^n + 7 is prime.
Numbers n such that digit 6 followed by n >= 0 occurrences of digit 0 followed by digit 7 is prime.
Numbers corresponding to terms <= 854 are certified primes.
a(23) > 2*10^5. - Robert Price, Sep 17 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 600...007.
FORMULA
a(n) = A103026(n-1) - 1.
EXAMPLE
6007 is prime, hence 2 is a term.
MATHEMATICA
For[n=1, n<=3000, n++, If[PrimeQ[60*10^n+7], Print[n]]] (Steinerberger)
PROG
(PARI) a=67; for(n=0, 1200, if(isprime(a), print1(n, ", ")); a=10*a-63)
(PARI) for(n=0, 1200, if(isprime(60*10^n+7), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard
AUTHOR
Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 06 2004
EXTENSIONS
a(13)-a(15) from Stefan Steinerberger, Feb 03 2006
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(18)-a(22) from Kamada link by Ray Chandler, Apr 30 2015
STATUS
approved