Re: I do not understand

*To*: mathgroup at smc.vnet.net*Subject*: [mg25446] Re: I do not understand*From*: "Allan Hayes" <hay at haystack.demon.co.uk>*Date*: Sun, 1 Oct 2000 02:44:28 -0400 (EDT)*References*: <8r180o$hin@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Borut, With ySer=Series[y[t],{t,0,3}] y'[t] /. y[t] -> ySer does nothing because y[t] does not appear in y'[t]: FullForm[y'[t]] Derivative[1][y][t] Mathematica regards Derivative[n][y] as the result of differentiating the *function* y n times. We can use y'[t] /. y -> Function[t, Evaluate[ySer]] Allan --------------------- Allan Hayes Mathematica Training and Consulting Leicester UK www.haystack.demon.co.uk hay at haystack.demon.co.uk Voice: +44 (0)116 271 4198 Fax: +44 (0)870 164 0565 "Borut L" <justmyname at email.si> wrote in message news:8r180o$hin at smc.vnet.net... > Hello, > > ySer=Series[y[t],{t,0,3}] > > If I apply this rule like this > (y[t]+y'[t])/.y[t]->ySer > both y[t] and y'[t] changes appropriately > > If I apply > y'[t]/.y[t]->ySer > nothing happens > > > Why is that? > > >