Services & Resources / Wolfram Forums / MathGroup Archive
-----

MathGroup Archive 2009

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: Inverse function warnings

  • To: mathgroup at smc.vnet.net
  • Subject: [mg99237] Re: [mg99164] Inverse function warnings
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Thu, 30 Apr 2009 06:25:23 -0400 (EDT)
  • Reply-to: hanlonr at cox.net

eqn = 3^(2 x) - 12 (3^x) + 27 == 0;

Quiet[Solve[eqn, x]]

{{x -> 1}, {x -> 2}}

Off[Solve::ifun]

Solve[eqn, x]

{{x -> 1}, {x -> 2}}

{Reduce[eqn, x, Reals] // ToRules}

{{x -> 1}, {x -> 2}}

{Reduce[{eqn, Element[x, Reals]}, x] // ToRules}

{{x -> 1}, {x -> 2}}

Reduce[eqn, x]

Element[C[1], Integers] && 
   (x == 1 + (2*I*Pi*C[1])/Log[3] || 
      x == 2 + (2*I*Pi*C[1])/Log[3])

This indicates that there are an infinite number of solutions. For any integer n

Simplify[eqn /. x -> 1 + 2 I*Pi*n/Log[3], Element[n, Integers]]

True

Simplify[eqn /. x -> 2 + 2 I*Pi*n/Log[3], Element[{n}, Integers]]

True


Bob Hanlon

---- davef <davidfrick2003 at yahoo.com> wrote: 

=============
When I execute this in Mathematica 7:

Solve[3^(2 x) - 12 (3^x) + 27 == 0, x]

I get this:

Solve::ifun: Inverse functions are being used by Solve, so some solutions may not be found; use Reduce for complete solution information. >> 

{{x->1},{x->2}}


1 amd 2 are proper solutions but is it possible to avoid the warning?

If I use Reduce:

Reduce[3^(2 x) - 12 (3^x) + 27 == 0, x]

I get a set of 1 and 2 added to some imaginary number terms that I don't quite understand.

I guess my question is: why would the use of inverse functions be so unreliable a solution as to necessitate a warning?  And in the interest of clean output, can the warning be supressed other than by deleteing the cell?  

Thanks




  • Prev by Date: Re: Simplify exponents in output
  • Next by Date: Re: NDSolve initial value problem
  • Previous by thread: Re: Inverse function warnings
  • Next by thread: Re: Inverse function warnings