[see also: as, because] Since f is compact, we have Lf=0.
Since f is compact, it follows that Lf=0. [Not: “Since f is compact, then Lf=0.”]
[Make sure the reader knows which part of a sentence beginning with Since is the premise, and which is the conclusion: instead of writing: “Since A=B, B=C, C=D, A=C'', write e.g.: Since A=B and B=C, and since moreover C=D, it follows that A=C.]
The argument is a variant of one in  and has been used several times since.