Because the propogator is actually in terms of the <i>differences</i> between the x and t values, we will write the difference between the x-vectors as <math>\Delta \mathbf{x}</math> and the difference between the times as <math>\Delta t</math>.
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Because the propogator is actually in terms of the <i>differences</i> between the x and t values, we will write the difference between the x-vectors as '''Δx''' and the difference between the times as <math>\Delta t</math>.