0.4146825098511116602...

This number is neither prime nor composite.

                                    0.41468 2509851111 6602481096 2215430770
8365774238 1379169778 6824541448 8640960619 3573341962 9004842847 5777939616

The Prime Constant. Ferenc Adorjan writes: "Decimal expansion of the representation of the sequence of primes by a single real in (0,1). Any monotonic integer sequence can be represented by a real number in (0,1) such a way that in the binary representation of the real, the n-th fractional digit is 1 iff n is in the sequence." Post explains: "take the characteristic function of the primes, concatenate the bits, interpret as a binary fraction, and convert to a decimal fraction." [Post]