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MathGroup Archive 1994

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Re: eval differential expressions

  • To: mathgroup at
  • Subject: Re: eval differential expressions
  • From: rubin at (Paul A. Rubin)
  • Date: Mon, 20 Jun 94 12:39:42 EDT

>Is there an easy way to evaluate differential expressions for various
>For example, if I have D[f[x],x]^2+f[x] in Mma, I'd like to evaluate it for
>various functions f[x]. This can be done via naming the expression and
>then letting f[x_]=<desired fn of x> and typing the expressions name
>upon which its evaluated - but this seems awfully clumbsy!
>Any suggestions?
Two possibilities:

1.  Make your expression an operator (on f):

    g[ f_ ] := D[f[x], x]^2 + f[x]

    g[ <your function here> ] or g /@ { <list of your functions here> }

2.  Name your expression and then use the replacement operator to substitute
    other functions for f:

    g = D[f[x], x]^2 + f[x]  (* f as yet unspecified *)

    g /. {f -> <your function here>}

* Paul A. Rubin                                  Phone: (517) 336-3509   *
* Department of Management                       Fax:   (517) 336-1111   *
* Eli Broad Graduate School of Management        Net:   RUBIN at MSU.EDU    *
* Michigan State University                                              *
* East Lansing, MI  48824-1122  (USA)                                    *
Mathematicians are like Frenchmen:  whenever you say something to them,
they translate it into their own language, and at once it is something
entirely different.                                    J. W. v. GOETHE

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