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I want to do the reverse of PowerExpand and convert 2*Log[x+y] to Log[(x+y)^2]. The rule:

 t_ Log[x_] -> Log[x^t]

works for a Log[y], a Log[Pi], Pi Log[a], 2 Log[a], a Log[2] but not for 2 Log[Pi], Pi Log[2], Pi Log[EulerGamma].

Entering:

Pi Log[Pi] /. Pi Log[Pi] -> Log[Pi^Pi]

Returns:

Pi Log[Pi]

instead of:

Log[Pi^Pi]
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up vote 4 down vote accepted

I think it's doing it and then simplifying back. Try :

Pi Log[Pi] /. {x_ Log[y_] -> Hold[Log[y^x]]}
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3  
If only appearance is important you could also use HoldForm. – Mr.Wizard Mar 24 '13 at 15:27

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