Underlined S and K: $\underline{S}$, $\underline{K}$

mathbb S and K: $𝕊$, $𝕂$

mathcal S and K: $\mathcal{𝒮}$, $\mathcal{𝒦}$

nonbold sigma, Sigma: $\sigma \Sigma$

boldsymbol sigma, Sigma: $\sigma \Sigma$

mathbf{sigma, Sigma}: $\sigma \Sigma$

{mathbf{sigma, Sigma}}: $\sigma \Sigma$