OFFSET
1,2
EXAMPLE
a(1)=1 because 1*3-1=2=prime and 1*3+1=4=composite; a(2)=8 because 8*3-1=23=prime and 8*3+1=25=composite; a(3)=12 because 12*3-1=37=prime and 12*3+1=39=composite.
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Juri-Stepan Gerasimov, Mar 14 2010
STATUS
approved