It is a 1-form $\omega$ such that the distribution that it generates (see dual description of the distribution) is a completely integrable distribution or involutive distribution (is it exactly the same in this context?). That is, the 1-form satisfies dual characterization involutiveness, or more specifically
$$ d\omega=\eta \omega $$or
$$ d\omega \wedge \omega=0. $$________________________________________
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Author of the notes: Antonio J. Pan-Collantes
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