0.5759599688929454396...

This number is neither prime nor composite.

                                            0.57595996 8892945439 6431633754
9249669250 6513967176 4923636006 4079866537 2551698868 5284364098 7209172618

Stephens' Constant: 0.57595996889294543964316337549249669... = Product[1 - p/(p^3-1)], where each p is a prime ≥ 2. [Post]