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For the following expression,

b'[x y/z]+z b''[x y/z]

I want to replace the argument of b such that the above expression becomes

b'[z]+z b''[z]

How can I do that ?

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    $\begingroup$ Note that if xy is intended to be the product of x and y then it should be entered as either x y (implicit product with a space) or as x*y (explicit product). $\endgroup$ – Bob Hanlon May 30 '16 at 14:21
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As a general case where you want to replace all the arguments,

(b'[x y/z] + z b''[x y/z]) /. Derivative[n_][b_][x_] :> Derivative[n][b][z]

z b''(z)+b'(z)

No matter what was your previous argument, it will be replaced by z.

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b'[xy/z] + z b''[xy/z] /. {xy/z -> z}

gives me an output of

b'[z]+z b''[z]

Is that what you meant?

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