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?
>
>
>

```

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