Silverman Constant

$\displaystyle \sum_{n=1}^\infty {1\over\phi(n)\sigma(n)}$	$\textstyle =$	$\displaystyle \prod_{p\rm {\ prime}} \left({1+\sum_{k=1}^\infty {1\over p^{2k}-p^{k-1}}\,}\right)$
	$\textstyle =$	$\displaystyle 1.786576459\ldots,$

where $\phi(n)$ is the Totient Function and $\sigma(n)$ is the Divisor Function.

References