\vspace{-1.6cm} The paper studies the relations between $\phi$-divergences and fundamental concepts of decision theory such as sufficiency, Bayes sufficiency, and LeCam's deficiency. A new and considerably simplified approach is given to the spectral representation of $\phi $-divergences already established in Österreicher and Feldman \cite{OestFeld} under restrictive conditions and in Liese and Vajda \cite{LiV06}, \cite{LiV08} in the general form. The simplification is achieved by a new integral representation of convex functions in terms of elementary convex functions which are strictly convex at one point only. Bayes sufficiency is characterized with the help of a binary model that consists of the joint distribution and the product of the marginal distributions of the observation and the parameter, respectively. LeCam's deficiency is expressed in terms of $\phi $-divergences where $\phi $ belongs to a class of convex functions whose curvature measures are finite and satisfy a normalization condition.
divergences, sufficiency, Bayes sufficiency, deficiency
62B05, 62B10, 62B15, 62G10