Re: Problems with Mathematica 8.0 Solve

• To: mathgroup at smc.vnet.net
• Subject: [mg114212] Re: Problems with Mathematica 8.0 Solve
• From: Daniel Lichtblau <danl at wolfram.com>
• Date: Sat, 27 Nov 2010 03:37:05 -0500 (EST)

```----- Original Message -----
> From: "leigh pascoe" <leigh at evry.inserm.fr>
> To: mathgroup at smc.vnet.net
> Sent: Friday, November 26, 2010 4:32:06 AM
> Subject: [mg114198] Problems with Mathematica 8.0 Solve
> Dear Mathgroup,
>
> Like many of you I have recently installed Mathematica 8.0 and have
> been
> exploring its new features. I have encountered the following problem
> using the Solve Function:
>
> Firstly in Mathematica 7.0
>
> I try to solve the following two simultaneous equations
> In[35]
> eq1 = (b + d + f)/x - (a + b)/(1 + x) - 2*(c + d + e)/(1 + 2*x + y) -
> (f + g)/(x + y) == 0
> eq2 = (e + g)/y - (c + d + e)/(1 + 2*x + y) - (f + g)/(x + y) == 0
>
> In[36}
> Timing[sols = Solve[{eq1, eq2}, {x, y}];]
> Out[36]
> {52.437, Null}
>
> A bit slow but it gives me the result I want. I then Simplify the
> solutions and evaluate them with a set of vectors of the constants
> {a,b,c,d,e,f,g}.
>
> subs = {{a -> 45, b -> 79, c -> 4, d -> 11, e -> 8, f -> 0, g -> 2},
> {a -> 95, b -> 85, c -> 9, d -> 13, e -> 6, f -> 0, g -> 1},
> {a -> 18, b -> 45, c -> 0, d -> 1, e -> 2, f -> 0, g -> 0}, {a -> 50,
> b -> 41, c -> 0, d -> 1, e -> 0, f -> 0, g -> 0},
> {a -> 17, b -> 61, c -> 1, d -> 9, e -> 14, f -> 0, g -> 1}, {a ->
> 68, b -> 60, c -> 10, d -> 7, e -> 3, f -> 2, g -> 0},
> {a -> 42, b -> 90, c -> 5, d -> 29, e -> 49, f -> 6, g -> 17}, {a ->
> 140, b -> 108, c -> 23, d -> 32, e -> 19, f -> 16, g -> 8}}
>
> nsols = Simplify[RootReduce[sols /. subs]]
>
> N[%]
>
> giving me the answers I want
>
> {{{y -> -2.18473, x -> 2.02599}, {y -> -0.0457403, x -> -0.53058}, {y
> ->
> 2.72299, x -> 1.70867}}, {{y -> -1.05434, x -> 1.00657}, {y ->
> -0.0244319, x -> -0.536953},
> {y -> 0.83139, x -> 0.871734}}, {{y -> -2.66667, x -> 2.66667}, {y ->
> -0.0183209, x -> -0.50458}, {y -> 12.1294, x -> 2.53236}}, {{y ->
> -0.84,
> x -> 0.84},
> {y -> 0., x -> -0.503749}, {y -> 0., x -> 0.833749}}, {{y -> -4.83381,
> x -> 4.69898}, {y -> -0.100097, x -> -0.530885}, {y -> 12.0561, x ->
> 3.63746}}, {{y -> -1.05985,
> x -> 0.917926}, {y -> -0.014617, x -> -0.542283}, {y -> 0.351798, x
> -> 0.797433}}, {{y -> -4.52487, x -> 3.85637}, {y -> -0.240225, x ->
> -0.548865}, {y -> 7.61935, x -> 2.24569}},
> {{y -> -1.46526, x -> 1.07904}, {y -> -0.0612374, x -> -0.561339}, {y
> ->
> 0.684794, x -> 0.770645}}}
>
> In Mathematica 8.0 this process hangs at the Solve command. I was
> forced
> to abort the evaluation after more than an hour with no result. The
> program seems to use only 25% of the available cpu (i.e. 1 of the 4
> available in the quadcore) and never arrives at a result. I thought
> that
> equation solving was much improved in the new version, so what is
> going on?
>
> Perhaps there is a much better way to get the results I want. I need
> to
> evaluate the solutions to the above equations for large numbers of
> data
> vectors. I would appreciate any suggestions. Thanks in advance.
>
> LP

We are not exactly sure why this worked better in earlier versions (we know where it gets into trouble in version 8). First are some suggestions from Adam Strzebonski, regarding your use of parametrized Solve.

"[...] mention that
for the (hopefully rare) cases when the new Solve method turns out
to be slower than the old one we still have the old method available.

In[3]:= Timing[sols = Solve[{eq1, eq2}, {x, y}, Method->"Legacy"];]
Out[3]= {15.4497, Null}

Another possibility

parallelSolve[args__] :=
ParallelTry[#[args] &, {Solve, Solve[##, Method -> "Legacy"] &}]

In[10]:= AbsoluteTiming[sol = parallelSolve[{eq1, eq2}, {x, y}];]
Out[10]= {30.6728358, Null}

Let me point out some other issues. First is that you will not save significant time if your goal is to get "nice" results. Specifically, you can solve relatively fast once you plug in those numeric values.

In[5]:= Timing[solns = Map[Solve[(exprs /. #) == 0] &, Take[subs, 4]];]
Out[5]= {0.889, Null}

But the reduction is relatively slow.

In[6]:= Timing[RootReduce[solns];]
Out[6]= {161.009, Null}

Also in version 8 you can readily suppress the radical solutions and then it will RootReduce quote quickly.

In[6]:= Timing[
solns = Map[Solve[(exprs /. #) == 0, Cubics -> False] &,
Take[subs, 4]];]
Out[6]= {0.063, Null}

In[9]:= Timing[RootReduce[solns];]
Out[9]= {0.032, Null}

If your end goal is the numeric solutions, I generally recommend NSolve.

In[10]:= Timing[Map[NSolve[exprs /. #] &, subs]]

Out[10]= {0.28, {{{y -> -2.18473, x -> 2.02599}, {y -> 2.72299,
x -> 1.70867}, {y -> -0.0457403, x -> -0.53058}}, {{y -> -1.05434,
x -> 1.00657}, {y -> -0.0244319, x -> -0.536953}, {y -> 0.83139,
x -> 0.871734}}, {{y -> 12.1294, x -> 2.53236}, {y -> -0.0183209,
x -> -0.50458}}, {}, {{y -> 12.0561,
x -> 3.63746}, {y -> -4.83381, x -> 4.69898}, {y -> -0.100097,
x -> -0.530885}}, {{y -> -1.05985,
x -> 0.917926}, {y -> -0.014617, x -> -0.542283}, {y -> 0.351798,
x -> 0.797433}}, {{y -> -4.52487, x -> 3.85637}, {y -> 7.61935,
x -> 2.24569}, {y -> -0.240225, x -> -0.548865}}, {{y -> -1.46526,
x -> 1.07904}, {y -> -0.0612374, x -> -0.561339}, {y -> 0.684794,
x -> 0.770645}}}}

Daniel Lichtblau
Wolfram Research

```

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