Author comment: as a fairly common convention, QED immediately after a particular statement means that the statement should be considered proven. Depending on the text, this may either be because the statement (Observation) is self-explanatory, or, the discussion in the text leading up to the statement is sufficient, or, whenever the final statement of a Theorem follows as a direct corollary of Lemmas previously proven in the text.
Author comment: as a fairly common convention, QED immediately after a particular statement means that the statement should be considered proven. Depending on the text, this may either be because the statement (Observation) is self-explanatory, or, the discussion in the text leading up to the statement is sufficient, or, whenever the final statement of a Theorem follows as a direct corollary of Lemmas previously proven in the text.