0.3739558136192022880...

This number is neither prime nor composite.

                                             0.3739558 1361920228 8054728054
3464164151 1162924860 6150042094 4280241735 0182040028 0823443043 1708725056

Decimal expansion of Artin's constant product, for p = prime: C = 1 - 1/(p^2-p). Note that Artin also conjectured that C is the fraction of primes p for which 1/p has decimal maximal period. [Post]