Rate distortion when side information may be absent

The problem is considered of encoding a discrete memoryless source when correlated side information may or may not be available to the decoder. It is assumed that the side information is not available to the encoder. The rate-distortion function is determined where is the distortion achieved with side information and is the distortion achieved without it. A generalization is made to the case of decoders, each of which is privy to its own side information. An appropriately defined -admissible rate for this general case is shown to equal when the side information sources satisfy a specified degradedness condition. Explicit results are obtained in the quadratic Gaussian case and in the binary Hamming case.