12.1 CLASSES OF NONSTATIONARY DATA12.2 PROBABILITY STRUCTURE OF NONSTATIONARY DATA12.3 NONSTATIONARY MEAN VALUES12.4 NONSTATIONARY MEAN SQUARE VALUES12.5 CORRELATION STRUCTURE OF NONSTATIONARY DATA12.6 SPECTRAL STRUCTURE OF NONSTATIONARY DATA12.7 INPUT/OUTPUT RELATIONS FOR NONSTATIONARY DATAPROBLEMSREFERENCES