Pattern

${\mu }_{111}^{abc}$

${\mu }_{112}^{abc}$ [3]

${\mu }_{123}^{abc}$

$abc$

$\frac{n_a}{n}$

$\frac{3n_a\left(n_a-1\right)}{n\left(n-1\right)}$

$\frac{n_a\left(n_a-1\right)\left(n_a-2\right)}{n\left(n-1\right)\left(n-2\right)}$

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

0

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

$\frac{n_a{n}_b\left(n_b-1\right)}{n\left(n-1\right)\left(n-2\right)}$

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

0

0

$\frac{n_a{n}_b{n}_c}{n\left(n-1\right)\left(n-2\right)}$