Re: assuming certain properties about variables
- To: mathgroup at smc.vnet.net
- Subject: [mg111513] Re: assuming certain properties about variables
- From: Murray Eisenberg <murray at math.umass.edu>
- Date: Tue, 3 Aug 2010 06:43:26 -0400 (EDT)
I don't see what $Assumptions has to do with the situation at hand, since using $Assumptions = t>0 does not change the result of leaving f[t] unevaluated; nor does either of Assuming[t>0, f[t]] Simplify[f[t], t>0] The only really workable thing I've seen posted in response to the O.P.'s query -- sorry, I cannot find any previous posting by you to MathGroup on this subject -- is Leonid Shifrin's [mg111421], using UpValues: t /: Positive[t] = True And I guess that, except for employing user-defined heads to create new types of objects, that's the closest one can come to declaring types of variables. Or am I missing something? On 8/2/2010 4:26 AM, Christoph Lhotka wrote: > 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. > -- Murray Eisenberg murray at math.umass.edu Mathematics & Statistics Dept. Lederle Graduate Research Tower phone 413 549-1020 (H) University of Massachusetts 413 545-2859 (W) 710 North Pleasant Street fax 413 545-1801 Amherst, MA 01003-9305