OFFSET
1,2
COMMENTS
Numbers n such that (650*10^n + 43)/9 is prime.
Numbers n such that digit 7 followed by n >= 0 occurrences of digit 2 followed by digit 7 is prime.
Numbers corresponding to terms <= 1785 are certified primes. For larger numbers see P. De Geest, PDP Reference Table.
a(13) > 200000. - Tyler Busby, Jan 14 2023
REFERENCES
Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.
LINKS
Patrick De Geest, PDP Reference Table - 727.
Makoto Kamada, Prime numbers of the form 722...227.
FORMULA
a(n) = A082711(n) - 2.
EXAMPLE
72227 is prime, hence 3 is a term.
PROG
(PARI) a=77; for(n=0, 1000, if(isprime(a), print1(n, ", ")); a=10*a-43)
(PARI) for(n=0, 1000, if(isprime((650*10^n+43)/9), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Robert G. Wilson v, Aug 18 2000
EXTENSIONS
Additional comments from Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 03 2004
Edited by N. J. A. Sloane at the suggestion of Andrew S. Plewe, Jun 08 2007
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
One more term added from PDP table and comments section updated by Patrick De Geest, Nov 02 2014
Edited by Ray Chandler, Nov 05 2014
a(12) from Tyler Busby, Jan 11 2023
STATUS
approved