All explained here:

* [MR0546622](https://mathscinet.ams.org/mathscinet-getitem?mr=546622) Pierre Deligne, _Valeurs de fonctions $L$ et périodes d'intégrales_,
with an appendix by N. Koblitz and A. Ogus. Proc. Sympos. Pure Math., XXXIII, Automorphic forms, representations and $L$-functions (Proc. Sympos. Pure Math., Oregon State Univ., Corvallis, Ore., 1977), Part 2, pp. 313–346, Amer. Math. Soc., Providence, R.I., 1979. [link to IAS copy](https://publications.ias.edu/node/379)