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Re: Delayed Derivative Operator


On 3 Oct 2007, at 15:26, ToddSmith wrote:

> Hi,
>  I would like to be able to have an equation
>
> eq := D[f[x],x]==5;
>
> And a substitution rule
>
> rule = f[x]->x^3;
>
> And perform the substitution
>
> eq/.rule
>
> And get back the equation 3x^2==5. But Mathematica sees f'[x] and
> doesn't do any substitutions. I tried a delayed assignment for eq but
> it didn't work. What can I do?
>
> -Thanks
>
>


One way:

eq = D[f[x], x] == 5;
eq /. f -> (#1^2 & )
2*x == 5

Andrzej Kozlowski


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