= 't Hooft symbol
{c}
{title2=$\eta^a_{\mu\nu},\ \bar\eta^a_{\mu\nu}$}
For the orientation $dx^1\wedge dx^2\wedge dx^3\wedge dx^4$, define $\eta^a_{ij}=\epsilon_{aij}$, $\eta^a_{i4}=\delta_{ai}$, and extend antisymmetrically. The three forms $\tfrac12\eta^a_{\mu\nu}dx^\mu\wedge dx^\nu$ are self-dual. Reversing the sign of the components involving index four gives the anti-self-dual symbols $\bar\eta$. They identify the two three-dimensional eigenspaces of the <Hodge star> on <differential two-forms> and give compact formulas for the <BPST instanton>.
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