Probability of Three Random Integers having no Common Divisor

Theorem
Let $a, b$ and $c$ be integers chosen at random.

The probability that $a, b$ and $c$ have no common divisor:
 * $\map \Pr {\map \perp {a, b, c} } = \dfrac 1 {\map \zeta 3}$

where $\zeta$ denotes the zeta function:
 * $\map \zeta 3 = \dfrac 1 {1^3} + \dfrac 1 {2^3} + \dfrac 1 {3^3} + \dfrac 1 {4^3} + \cdots$

The decimal expansion of $\dfrac 1 {\map \zeta 3}$ starts:
 * $\dfrac 1 {\map \zeta 3} = 0 \cdotp 83190 \, 73725 \, 80707 \ldots$