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Re: Getting rid of ProductLog

  • To: mathgroup at
  • Subject: [mg49258] Re: Getting rid of ProductLog
  • From: Bill Rowe <readnewsciv at>
  • Date: Fri, 9 Jul 2004 02:26:37 -0400 (EDT)
  • Sender: owner-wri-mathgroup at

On 7/8/04 at 2:51 AM, robert.hulme at (Robert Hulme) wrote:

>Solve[a^b - b == c, b]

>Which gives me:

>                         Log[a]
>            ProductLog[-(------)]
>                            c
>                           a
>{{b -> -c - ---------------------}}
>                   Log[a]

>The problem with this is that I need the solution to use normal
>'primitive' (if thats the right word) math functions as I need the
>formula for a computer program.

>With ProductLog being an internal Mathematica function I cant
>therefore use this rearrangement.

>What can I do so that there is no ProductLog in there? 

You can't. There is no closed form solution without the ProductLog function or the equivalent with a different name. If you are trying to write program that doesn't depend on Mathematica at runtime and need the solution to a^b - b == c, you will have to either write your own numeric solver or a routine to implement the ProductLog function.
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