OFFSET
0,4
COMMENTS
If n<m and r=10^n*a(m)*a(n)-1 is prime then r has at least n+1
representations of the form p*q-(p+q)where p & q are prime.
For n>3, 7 divides a(n). a(10) is greater than 4*10^10.
LINKS
Prime Puzzles Two Bergot questions
EXAMPLE
28+1, 280+1, 2800+1, 28000+1, 280000+1 & 2800000+1 are prime and 28 is the smallest such number so a(5)=28.
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Farideh Firoozbakht, Mar 15 2009, Mar 27 2009
EXTENSIONS
a(11)-a(13) from Don Reble, Jul 06 2022
STATUS
approved