Re: assuming certain properties about variables

*To*: mathgroup at smc.vnet.net*Subject*: [mg111467] Re: assuming certain properties about variables*From*: Christoph Lhotka <christoph.lhotka at univie.ac.at>*Date*: Mon, 2 Aug 2010 07:06:05 -0400 (EDT)

hello, there is the symbol $Assumptions exactly for this purpose (please see my previous response for a working definition of f, such that it will return t, whenever it is assumed to be positive)... best, christoph On 01/08/2010 10:57, Murray Eisenberg wrote: > But once you put the restriction on the argument of f, you've told > Mathematica not to carry out the evaluation of f unless the input > supplied is actually positive. > > If now t is just a symbol, then it is not positive (and not negative, > either) -- it's just a symbol. So why would you expect to be able to > tell Mathematica that it's positive? In general, Mathematica variables > don't really have types. > > If it's just the particular symbol t that you want to supply to f, then > you could do this (I'm changing your function definition for clarity) -- > not that you probably want SetDelayed (:=) instead of Set (=): > > f[x_?Positive] := x^2 > f[t] = t^2; > > f[3] > 9 > f[t] > t^2 > t = -5; > f[t] > f[-5] > > f > > > On 7/31/2010 2:40 AM, Benjamin Hell wrote: > >> let's say I have defined the following function: >> f[x_?Positive] = x >> Now I want to evaluate f with a variable t: >> f[t] >> As mathematica knows nothing about t, the output is f[t] instead of t. >> >> How can I tell mathematica, that t should be a positive number so that >> Positive[t] evaluates true and then f[t] evaluates to t? >> Of course this is just an example, which should present what I would >> like to know. >> >