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?
>
>
>