2.1 Discrete-time signals:sequences 2.1.1 Definition 2.1.2 Classification of sequence 2.1.3 Basic sequences 2.1.4 Period of sequence 2.1.5 Symmetry of sequence 2.1.6 Energy of sequence 2.1.7 The basic operations of sequences 2.2 Discrete-time system 2.2.1 Definition:input-output description of systems 2.2.2 Classification of discrete-time system 2.2.3 Linear time-invariant system(LTI) 2.2.4 Linear constant-coefficient difference equation 2.2.5. Direct implementation of discrete-time system 2.3 Frequency-domain representation of discrete-time signal and system 2.3.1 definition of fourier transform 2.3.2 frequency response of system 2.3.3 properties of fourier transform