Kurepa's left-factorial conjecture

For each integer $n\geq 2$, let the left factorial $\mathbf{!n}$ be the sum of the factorials below $n$, and let $n!$ denote the usual factorial. Kurepa's conjecture. The two factorials have greatest common divisor

$$\gcd(\mathbf{!n},n!)=2.$$

This is a classical divisibility conjecture concerning the arithmetic relationship between the left factorial and the ordinary factorial; its resolution is not established in the supplied source.