5.8154599887223282940...

This number is neither prime nor composite.

                   5. 8154599887 2232829407 3007970995 6593490083 3301145...

The smallest positive real solution to x^2/2! - x^3/3! + x^5/5! - x^7/7! + x^11/11! - ... = 0. [Hartley]