i(Z)l>≥ I>i(z+△Z u(②)也 u(Z+△Z） Z+△Z L△Z RAZ C△Z G△Z . e+△=t0+正 置 csngs i(z) i(z+ u(z) u(z+ z z+ z z) z) L z R z C z G z ( ) ( ) ( ) ( ) z z i i z z t i z t z z v v z z t v z t × D ¶ ¶ + D = + × D ¶ ¶ + D = + , , ,