MathGroup Archive 2009

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

Search the Archive

FindInstance over Integers

  • To: mathgroup at
  • Subject: [mg101077] FindInstance over Integers
  • From: "zac.ernst" <zac.ernst at>
  • Date: Tue, 23 Jun 2009 07:07:11 -0400 (EDT)

Hello --

I'm working on a project which requires the program to determine
whether a given set of equalities and inequalities is satisfiable over
the integers.  Of course, this problem is not decidable in general.
But I just need a "good enough" function that will at least report
that a set of constraints is satisfiable only if it actually is, and
work across some fairly easy cases.  FindInstance is the obvious
function to use, but it seems to fail for constraints that are very
easy to satisfy.  The simplest example I can come up with is this:

FindInstance[x == 2^y && x > 2, {x, y}, Integers]

Clearly, {x->4, y->2} would satisfy these constraints, but Mathematica
reports that "the methods available to FindInstance are

It may be relevant that if I change "x > 2" to "x > 1", then
Mathematica has no problem finding a solution.

Are there any workarounds or alternatives to this approach?

Thanks very much,

  • Prev by Date: Re: Two questions about modification of a matrix.
  • Next by Date: Re: Ansatz?
  • Previous by thread: Re: exporting coordinate triplets out of
  • Next by thread: Re: FindInstance over Integers