Pattern

${\mu }_{11}^{ab}$

${\mu }_{12}^{ab}$

$ab$

$\frac{n_a}{n}$

$\frac{n_a\left(n_a-1\right)}{n\left(n-1\right)}$

$a\mathrm{<mo>\; |\; </mo>}b$

0

$\frac{n_a{n}_b}{n\left(n-1\right)}$