OFFSET
1,7
EXAMPLE
a(9)=2 because f(9,2)=12223242526272829 is prime number and f(9,0)=10203040506070809 and f(9,1)=11213141516171819 are not.
PROG
(Magma) a:=[]; for n in [1..86] do if (n eq 1) or (n mod 2 eq 0) or (n mod 5 eq 0) then a[n]:=-1; else k:=0; while true do s:=IntegerToString(1); for i in [2..n] do s*:=IntegerToString(k)*IntegerToString(i); end for; if IsPrime(StringToInteger(s)) then a[n]:=k; break; end if; k+:=1; end while; end if; end for; a; // Jon E. Schoenfield, Nov 03 2013
CROSSREFS
KEYWORD
sign,base
AUTHOR
Farideh Firoozbakht, Jun 13 2003
EXTENSIONS
Missing terms added by Jon E. Schoenfield, Nov 03 2013
STATUS
approved