Invitation: celebrating 50 years of OEIS, 250000 sequences, and Sloane's 75th, there will be a conference at DIMACS, Rutgers, Oct 9-10 2014.
1,1
Also numbers n such that (4*10^(n+1)+41)/9 is prime.
Table of n, a(n) for n=1..7.
Index entries for primes involving repunits
Do[ If[ PrimeQ[40*(10^n - 1)/9 + 9], Print[n]], {n, 5000}]
Sequence in context: A154775 A210418 A167454 * A105288 A242915 A066684
Adjacent sequences: A056680 A056681 A056682 * A056684 A056685 A056686
hard,nonn
Robert G. Wilson v, Aug 10 2000
approved