# Equating coefficients in a non-power-series equation

How can I use Mathematica to equate coefficients in a non-power-series equation?

For example, I would like to take an equation like the following:
afx+b2fxfy+chfx=fx+eafx+3fxfy+2bhfxaf_x+\frac{b}{2}f_xf_y+chf_x=f_x+e^af_x+3f_xf_y+2bhf_x
and produce the following system:
a=1+eaa=1+e^a
b2=3\frac{b}{2}=3
c=2bc=2b
EDIT: This is a rather small example. If possible, I would prefer a solution that requires minimal human inspection of the original equation. The equations I will be working with will have many, perhaps hundreds of partial derivative terms, and it would be unfeasible to do things like individually pick them out. Ideally, I would like to specify only the unknowns I am interested in (in this case, {a, b, c}) and let Mathematica take it from there.

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1

You can subtract rhs from lhs, use ‘Variables’, remove all that are not ‘FreeQ’ of ‘{a,b,c}’, and then ‘SolveAlways[lhs-rhs, remaining variables]’.
– Daniel Lichtblau
Dec 29 ’13 at 21:22

SolveAlways[ a fx + b fx fy/2 + c h fx – (fx + Exp[a] fx + 3 fx fy + 2 b h fx) == 0, {fx, fy, h}] gives me SolveAlways::tdep: The equations appear to involve the variables to be solved for in an essentially non-algebraic way.
– David Zhang
Dec 29 ’13 at 22:43

Ah. I was hoping it would handle that a=1+e^a. Another possibility is to find the fx etc. vars progrmatically, mult iply all by a new variable, say t. Now take a Series in t. The coeffs should be the things you want to equate to zero. (Traveling sans Mathematica so cannot test this in detail).
– Daniel Lichtblau
Dec 30 ’13 at 1:13

Actually, your first method works if I use another similar equation, like SolveAlways[b fx fy + c h fx == 3 c fx fy + (a + b + 2 c) h fx, {fx, fy, h}]. I assume the issue arises from the equation a=1+eaa=1+e^a, which Mathematica doesn’t seem to like for being non-algebraic.
– David Zhang
Dec 30 ’13 at 7:37

Is there a way to tell Mathematica not to try to solve the equations produced by SolveAlways, but to simply return them?
– David Zhang
Dec 30 ’13 at 11:18

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2

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Also there is MonomialList.

coefficientRelations[expr_, params_] := Module[
{vars},
vars = DeleteCases[Variables[expr],
vv_ /; InternalDependsOnQ[vv, params]];
MonomialList[expr, vars] /. Thread[vars -> 1]
]

Your example is then as follows.

expr = a fx + b fx fy/2 +
c h fx – (fx + Exp[a] fx + 3 fx fy + 2 b h fx);
parameters = {a, b, c};

coefficientRelations[expr, parameters]

(* ut[104]= {-3 + b/2, -2 b + c, -1 + a – E^a} *)

Daniel, would you describe the function of InternalDependsOnQ please?
– Mr.Wizard♦
Jan 5 ’14 at 8:50

Interestingly, I cannot find anything about DependsOnQ in the Documentation Center. At first, I thought it might be something like a complement of FreeQ, but there are cases in which they disagree. For example, FreeQ[g[x], g] is False whereas Internal`DependsOnQ[g[x], g] is True.
– David Zhang
Jan 5 ’14 at 9:03

1

@DavidZhang just explained it. That is to say, it is a more nuanced version of FreeQ, that decides there is no dependency between e.g. x and x[1]. Also the second arg can be a list, rather than an Alternatives construct. The author no longer recalls the reason for this difference. Possibly indifference, at the time. Or maybe just that DependsOnQ is not pattern-based.
– Daniel Lichtblau
Jan 5 ’14 at 22:48

Probably (ok, most likely) not the most elegant way. But here we go: May be you can let x=fxx=f_x and y=fyy=f_y and use CoefficientList to help.

ClearAll[x, y, a, b, c, h, r, lhs, rhs]
fx = D[f[x, y], x];
fy = D[f[x, y], y];
expr = a fx + b/2 fx fy + c h fx == fx + Exp[a] fx + 3 fx fy + 2 b h fx

expr = expr /. {fx -> x, fy -> y}

lhs = Last@CoefficientList[expr /. (lhs_ == rhs_) :> lhs, {x, y}];
rhs = Last@CoefficientList[expr /. (lhs_ == rhs_) :> rhs, {x, y}];
(r = MapThread[Equal, {lhs, rhs}]) // TableForm

One small part left

r2 = r[[1]] /. (h any_ + __) :> h any;
r2 /. (any1_ h == any2_ h) :> any1 == any2
Inner[Subtract, r[[1]], r2, Equal]

Thanks. This does work, but it seems this requires quite a bit of inspection. For what I’m working on, I will need to deal with many partial derivative terms (perhaps hundreds) and picking them out by hand this way would be unfeasible. Is there any more automated solution?
– David Zhang
Dec 29 ’13 at 18:23

@DavidZhang as I said, it is not the most automated method, but unless some Mathematica parsing expert steps in and can see a more direct way, I thought this might be a start. The transformation you are asking for is not a common one, and I could not think of a more direct way myself…..
– Nasser
Dec 29 ’13 at 20:06