OFFSET
1,1
EXAMPLE
a(1)=5=prime because 2^5-1=31=prime and 2*5-1=9=nonprime; a(2)=13=prime because 2^13-1=8191=prime and 2*13-1=25=nonprime.
CROSSREFS
KEYWORD
nonn
AUTHOR
Juri-Stepan Gerasimov, Feb 03 2010, Feb 06 2010
EXTENSIONS
Corrected by Juri-Stepan Gerasimov, Feb 06 2010
Definition edited by Emeric Deutsch, Feb 13 2010
Extended by Emeric Deutsch, Feb 13 2010
STATUS
approved