"solve" doesn't return anything

[dennis]

Hi,

when i run "solve(['x-1', 'x-2'])" there is nothing returned. Shouldn't it return an empty list [], as there are no solutions?

[David Joyner]

sage: x = var("x")
sage: solve([x-1, x-2],[x])
[]

>
> --
> You received this message because you are subscribed to the Google Groups
> "sympy" group.
> To view this discussion on the web visit
> https://groups.google.com/d/msg/sympy/-/97WBZB6-hAsJ.
> To post to this group, send email to sy...@googlegroups.com.
> To unsubscribe from this group, send email to
> sympy+un...@googlegroups.com.
> For more options, visit this group at
> http://groups.google.com/group/sympy?hl=en.

[David Joyner]

On Thu, Feb 16, 2012 at 8:03 PM, David Joyner <wdjo...@gmail.com> wrote:
> On Thu, Feb 16, 2012 at 7:39 PM, dennis <dennis...@googlemail.com> wrote:
>> Hi,
>>
>> when i run "solve(['x-1', 'x-2'])" there is nothing returned. Shouldn't it
>> return an empty list [], as there are no solutions?
>
> sage: x = var("x")
> sage: solve([x-1, x-2],[x])
> []

Please, ignore that reply of mine. I thought you were asking about Sage!

[Chris Smith]

The docstring notes:

* when the system is linear

* with a solution

>>> solve([x - 3], x)
{x: 3}
>>> solve((x + 5*y - 2, -3*x + 6*y - 15), x, y)
{x: -3, y: 1}
>>> solve((x + 5*y - 2, -3*x + 6*y - 15), x, y, z)
{x: -3, y: 1}
>>> solve((x + 5*y - 2, -3*x + 6*y - z), z, x, y)
{x: -5*y + 2, z: 21*y - 6}

* without a solution

>>> solve([x + 3, x - 3])

So you are dealing with a linear system that has no solution. An empty
list might be returned if there were no solutions satisfying the
requirements (assumptions, etc...) but for this system there is no
solution so None is returned.

[Aaron Meurer]

So would it be accurate to say that None means that we know that there
are no solutions and [] means that we just didn't find any?

Aaron Meurer

[Chris Smith]

For a multi-equation system, yes.