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Re: Strange result of solve
Posted:
Mar 11, 2014 3:17 AM


Reduce is generally more powerful than Solve and indicates that the expression is True, i.e., valid for all integers, x.
Reduce[5 x==0,x,Modulus>5]
True
In your second example, the result of {{}} does not mean that every x is OK but rather that there is no solution for x.
Attributes[Solve]
{Protected}
Since Solve does NOT have either the attribute HoldFirst or HoldAll, your second example is equivalent to
Solve[True,x,Modulus>5]
{{}}
That is, there is no solution for x since x does not appear in the expression. You can verify this with
Solve[0 x==0,x,Modulus>5]//Trace
{{{0 x,0},0==0,True},{Modulus>5,Modulus>5},Solve[True,x,Modulus>5],{{}}}
Bob Hanlon
On Mon, Mar 10, 2014 at 4:41 AM, Wiel Aerts <wiel.aerts@xs4all.nl> wrote:
> > > I cannot really understand the different results of the following: > > Solve[ 5 x == 0, x, Modulus> 5 ] > result: {{x> 0}} > > and: > > Solve[ 0 x == 0, x, Modulus> 5 ] > result: {{}} > > The last means that every x is OK. Which I think is true. But I would > expect the same result for the first case. > > What am I missing? > > Wiel > > I'm using MTM v9. > > >



