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I have the following equation:

$$f(x,y)+3=0$$

Where $$f(x,y):=2 \cos(\sqrt{3}y)+4\cos(\frac{3}{2}x)\cos(\frac{\sqrt{3}}{2}y)$$

My goal is to solve the first equation, i.e. find the points (x,y) such that $f(x,y)=-3$.

I already know that there are infinite solutions periodically repeated. In particular the points in the xy plane that satisfy the equation are located on the vertices of a honeycomb structure (many hexagons repeated, like in graphene). For example the following points are possible solutions: $$(x,y)=(\frac{2\pi}{3},\frac{2\pi}{3 \sqrt{3}})$$ or

$$(x,y)=(\frac{2\pi}{3},-\frac{2\pi}{3 \sqrt{3}})$$

or also

$$(x,y)=(0,\frac{4\pi}{3 \sqrt{3}})$$

However when I try to obtain this in Mathematica by

Solve[f[x, y] + 3 == 0, {x, y}]

I get a very complicated expression, where y is expressed in terms of x. So how can I obtain with Mathematica the nice solutions I wrote above?

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2 Answers 2

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  • Reduce work.
f[x_, y_] = 2 Cos[Sqrt[3] y] + 4 Cos[3/2 x] Cos[Sqrt[3]/2 y];
Reduce[f[x, y] == -3, {x, y}, Reals]
Solve[f[x, y] + 3 == 0, {x, y}, Reals, Method -> Reduce]
Reduce[{f[x, y] + 3 == 0, -5 <= x <= 5, -5 <= y <= 5}, {x, y}, Reals]

enter image description here

  • We can also find some solutions by FindInstance.
f[x_, y_] = 2 Cos[Sqrt[3] y] + 4 Cos[3/2 x] Cos[Sqrt[3]/2 y];
FindInstance[f[x, y] + 3 == 0, {x, y}, Reals, 5]
f[x_, y_] = 2 Cos[Sqrt[3] y] + 4 Cos[3/2 x] Cos[Sqrt[3]/2 y];
pts = {x, y} /. 
  FindInstance[{f[x, y] + 3 == 0, -5 <= x <= 5, -5 <= y <= 5}, {x,
     y}, Reals, 10]

enter image description here

We can view the solution: the points is the intersection points of z=f[x,y] and z=-3,in fact, the tangent point of the z=f[x,y] and z=-3.

Show[Plot3D[{f[x, y], -3}, {x, -5, 5}, {y, -5, 5}, Mesh -> None, 
  Boxed -> False, PlotStyle -> {Opacity[.2]}, Axes -> False], 
 Graphics3D[{Red, AbsolutePointSize[10], 
   Point[PadRight[#, 3, -3] & /@ pts]}]]

enter image description here

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  • $\begingroup$ Thanks this is great! I was wondering, how does the procedure change if now the f function contains also other parameters, and I want to find the (x,y) solutions as a function of those parameters as well? Because if I try the same way I either get an error, or Mathematica gives a solution for the parameters as well (so it treats them as new variables). $\endgroup$
    – Mathew
    Aug 25, 2022 at 14:09
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Clear["Global`*"]

f[x_, y_] = 2 Cos[Sqrt[3] y] + 4 Cos[3/2 x] Cos[Sqrt[3]/2 y];

The function is periodic in both x and y

fp = FunctionPeriod[f[x, y], {x, y}]

(* {(4 π)/3, (4 π)/Sqrt[3]} *)

Restrict the range of interest by adding constraints, and Solve is more capable when given the option Method -> Reduce.

sol = Solve[{f[x, y] + 3 == 0, -fp[[1]]/2 <= x <= fp[[1]]/2, 
  -fp[[2]]/2 <= y <= fp[[2]]/2}, {x, y}, Method -> Reduce]

enter image description here

Show[
 Plot3D[f[x, y] + 3,
  {x, -fp[[1]]/2, fp[[1]]/2}, {y, -fp[[2]]/2, fp[[2]]/2},
  PlotStyle -> Opacity[0.67]],
 Graphics3D[{Red, AbsolutePointSize[6], Point[{x, y, 0} /. sol]}]]

enter image description here

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