Is it possible to define unambiguously retarded and advanced Green's function(adsbygoogle = window.adsbygoogle || []).push({});

in spacetime without timelike Killing vector. Most often e.g. retarded Green

function [tex]G_R(t,\vec{x},t',\vec{x}') [/tex] is defined to be 0 unless t'<t

but maybe one can express this condition using only casual structure

(without time coordinate)?

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# Green's function for Klein-Gordno equation in curved spacetime

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