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Re: Derivatives in replacement rules

  • To: mathgroup at smc.vnet.net
  • Subject: [mg125776] Re: Derivatives in replacement rules
  • From: Rodrigo Murta <murta at rodrigomurta.com>
  • Date: Mon, 2 Apr 2012 04:22:48 -0400 (EDT)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <jl3v1b$3mi$1@smc.vnet.net>

On Mar 30, 6:39 am, Yi Wang <tririverwan... at gmail.com> wrote:
> Hi, all,
>
> When I am using the following replacement rule:
>
> b[t] c[t] a'[t] /. b_ a_'[t] -> D[b, t]
>
> I would expect to get D[b[t]c[t], t] = b'[t]c[t] + c'[t]b[t]. However,
> I got 0 actually. To debug, when I set
>
> b[t] c[t] a'[t] /. b_ a_'[t] -> b
>
> I get b[t] c[t]. Thus pattern matching works and it appears D[b,t]
> should be non-zero.
>
> Any ideas how I can get D[b[t]c[t], t] in the above pattern
> replacement? Thanks!

Hi Yi
Use the dalayed rule :> instead of ->

b[t] c[t] a'[t] /. x_ a_'[t] :> D[x, t]

and you will get your expected result

Best regards!
Murta




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