If we again think of the example of x(t) as a tape recording, then x(2t) is that recording played at twice the speed and x(t/2) is the recording played at half spead An important fact is that any signal can be broken into a sum of two signals one of which is even and one of which is odd Fourier series representation of peridic signals causality 因果关系 The cotinuous-time Fourier Transform DTFT discrete time fourier transform DFT discrete fourier tansform FFT fast fourier transform T0 refers to the fundamental period,smallest positive value steps,impulses,unit steps and unit impulses exponential,growing expoxential,decaying exponential sampling theorem stable/unstable system degrade 衰减,enhance,restore恢复,compensate the time interval t1<=t<=t2 time shift,shift to the left by one unit along the t axis real part,imaginary part converge 会聚 收敛 eq.equation sinusoidal rectangular summation m and n do not have any factors in common arrow 数轴