Contents Chapter III. A study, based on the mean-value formula, of spectral decompositions for second-order elliptic operators § 1. Basic notation and definitions § 2. Properties of fundamental functions § 3. An estimate of the remainder term of the spectral function in the L2 metric and its consequences § 4. A uniform estimate of the remainder term of the spectral function § 5. The general case of second-order elliptic operators References