Re: Real roots and other assumptions...

*To*: mathgroup at smc.vnet.net*Subject*: [mg19939] Re: Real roots and other assumptions...*From*: "Kevin J. McCann" <kevin.mccann at jhuapl.edu>*Date*: Wed, 22 Sep 1999 04:11:19 -0400*Organization*: Johns Hopkins University Applied Physics Lab, Laurel, MD, USA*References*: <7s3p1u$ck5@smc.vnet.net> <7s796h$htf@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

This does work Select[{(1 - E)^(1/3)}, Im[#1] == 0 & ] {} as does this: Select[{(2. + I)*(2. - I)}, Im[#1] == 0 & ] which gives the odd-looking result {5.+0.I} and the original problem: Solve[(x - 1)*(x^2 + 1) == 0, x] {{x -> -I}, {x -> I}, {x -> 1}} but Select[x /. Solve[(x - 1)*(x^2 + 1) == 0, x], Im[#1] == 0 & ] {1} -- Kevin J. McCann Johns Hopkins University APL Allan Hayes <hay at haystack.demon.co.uk> wrote in message news:7s796h$htf at smc.vnet.net... > > Janus Wesenberg <jaw at imf.au.dk> wrote in message > news:7s3p1u$ck5 at smc.vnet.net... > > Hi, > > I keep encountering problems of the following type when using mathematica: > > I want to solve some equation(s) under some assumptions about the > unknown(s), > > e.g. find the real roots of (x-1)(x^2+1). > > I've tried Solve[{Im[x]==0,(x-1)(x^2+1)==0},x]] for the above problem, but > that > > doesn't get me anywhere. > > > > Is there a general way to let Mathematica know about such additional > bounds as > > non-complexness etc? -- if so I would be happy to know it! > > > > Janus Wesenberg > > Student of Physics. > > > > PS. I'm using Mathematica from a HP-UX 10 system, and the notebook > interface > > have grave difficulties handling large expressions (they scrambled to > complete > > nonsense). The local system administrator just says "Use the text access", > but > > does anyone know how to make the notebook interface work? > > > > Janus, > > Cases[Solve[(x - 1)(x^2 + 1) == 0, x], > s_ /; FreeQ[s, _Complex]] > > {{x -> 1}} > > But there could be problems with precision and also with rules like > > x -> (a - I)(a + I) (*which may, depending on a, give real > solutions but would be excluded*) > > x-> (1-e)^{1/3) (* gives a complex solution, but would not be > excuded*) > <snip-snap>