$10\mathrm{.}8$

For integers n $>=1,$ let $\backslash$phi $($n$)$ denote Euler$$s totient function. Prove that the sum

$1<=$d$|$n $\backslash$phi $($d$)$ equals n$.($For example: $\backslash$phi $(1)=1,\backslash$phi $(2)=1,\backslash$phi $(3)=2,\backslash$phi $(4)=2,\backslash$phi $(6)=2,$

$\backslash$phi $(12)=4$