Re: Getting rid of ProductLog
- To: mathgroup at smc.vnet.net
- Subject: [mg49243] Re: Getting rid of ProductLog
- From: John Doty <jpd at whispertel.LoseTheH.net>
- Date: Fri, 9 Jul 2004 02:26:13 -0400 (EDT)
- References: <ccisbq$4de$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
You might take a look at http://en.wikipedia.org/wiki/Lambert's_W_function ProductLog, or W, *is* elementary, in the sense that it cannot be expressed in terms of more common elementary functions. If all you want is a numerical solution, it may be easier and faster to apply a numerical root finding algorithm to your original equation rather than attempt to numerically evaluate a symbolic solution. Robert Hulme wrote: > Hi, > > Could someone please help me? > > I'm not a mathematician, but rather a programmer - I'm trying to use > Mathematica to rearrange a formula for me. > > I'm trying: > > Solve[a^b - b == c, b] > > Which gives me: > > Out[3]//TextForm= > 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? Please go easy > on me as I'm not a math major :0) or that au fait with Mathematica. > > If it helps both a and b are always positive. > > Many thanks > -Rob >