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MathGroup Archive 2000

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Re: Differential operators, Help

  • To: mathgroup at smc.vnet.net
  • Subject: [mg25364] Re: [mg25332] Differential operators, Help
  • From: BobHanlon at aol.com
  • Date: Sun, 24 Sep 2000 03:01:32 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

In a message dated 9/23/2000 4:03:07 AM, wkb at ansto.gov.au writes:

>This should be relatively easy, but after several tries I have not been
>able
>to do it.
>
>I want define a differential operator in the following way. Let Dx and
>Dx2
>denote the operators for first and second order differentiation with respect
>to x. I want P to be an operator which depends on x,  Dx and Dx2.
>
>For example, with
>
>                  P = Dx2 + x Dx
>
>I want P[f[z],z] = f''[z] + z f'[z].
>
>This much I can do, but I cannot find a method which also gives the
>following result,
>
>P[ P[f[z],z], z]  -> (f''[z] + z f'[z])''+ z (f''[z] + z f'[z])'
>
>Or more generally, if I have two such operators P and Q I want the correct
>result from expressions such as
>
>      P[ Q[f[x],x], x]
>

P[expr_, x_Symbol:x] := D[expr, {x, 2}] + x*D[expr, x];

Q[expr_, x_Symbol:x] := D[expr, {x, 2}] - 2x^2*D[expr, x];

P[f[z], z]

z*Derivative[1][f][z] + Derivative[2][f][z]

P[P[f[z], z], z] // Simplify

z*Derivative[1][f][z] + (2 + z^2)*Derivative[2][f][z] + 
  2*z*Derivative[3][f][z] + Derivative[4][f][z]

D[f''[z] + z f'[z], {z, 2}] + z *D[f''[z] + z f'[z], z] == % // Simplify

True

P[ Q[f[x]]]

-4*Derivative[1][f][x] - 8*x*Derivative[2][f][x] - 
  2*x^2*Derivative[3][f][x] + 
  x*(-4*x*Derivative[1][f][x] - 2*x^2*Derivative[2][f][x] + 
    Derivative[3][f][x]) + Derivative[4][f][x]


Bob Hanlon


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