Invariance principle and bilinear relations of the radiative transfer theory .1.

This is the first of two papers devoted to physical interpretation of Rybicki's quadratic and bilinear relations. The fundamental equations obtained on the basis of Ambartsumian's invariance principle and regarded as its extension to all depths in the atmosphere, imply that the Q and R relations have with more general structures than those known up to the present. These equations allow a simple probabilistic interpretation. Some bilinear relations are derived to connect the transfer problems of different sorts. For the sources distributed in the semi-infinite atmosphere by exponential law, the separate Q and R relations are obtained. It is shown that the two-point quadratic Q relation may be derived directly on the base of physical considerations used in the invariance technique.