f(t)=LIF(SI=L-Irk K2 s-Ps-p2 L[]+L["2-]+…+L[ s-p ke+k2e+…k,"t≥0 闪四 西南交通大学西南交通大学 ( ) [ ( )] [ ] 2 2 1 1 1 1 n n s p k s p k s p k f t F s - + + - + - = = L - L - L L [ ] L [ ] L [ ] 1 2 1 2 1 1 1 n n s p k s p k s p k - + + + - + - = - - L - 0 1 2 = k1 e + k2 e + k e t ³ p t n p t p t L n