A062576 - OEIS

A062576

Numbers k such that 10^k - 9^k is prime.

2, 3, 7, 11, 19, 29, 401, 709, 2531, 15787, 66949, 282493

COMMENTS

Terms > 1000 are often only strong pseudoprimes.

EXAMPLE

10^2 - 9^2 = 100 - 81 = 19, which is prime, hence 2 is in the sequence.

10^3 - 9^3 = 1000 - 729 = 271, which is prime, hence 3 is in the sequence.

10^4 - 9^4 = 10000 - 6561 = 3439 = 19 * 181, which is not prime, hence 4 is not in the sequence.

MATHEMATICA

Select[Range[1000], PrimeQ[10^# - 9^#] &] (* Alonso del Arte, Sep 06 2013 *)