Skip to main content

Solve doesn't give an answer with system of equations


I have to solve the following system of equations:


Solve[{k*Sech[d]*Cos[b]/Sqrt[A^2 + B^2] == 1, 
k*Sech[d]*Sin[b]/Sqrt[A^2 + B^2] == 0, A*k*Tanh[d]/(A^2 + B^2)+c1 == 1,

B*k*Tanh[d]/(A^2 + B^2)+c2 == 0, k*Sech[k + d]*Cos[a + b]/Sqrt[A^2 + B^2] == 1,
k*Sech[k + d]*Sin[a + b]/Sqrt[A^2 + B^2] == 0.1,
A*k*Tanh[k + d]/(A^2 + B^2)+c1 == 1.1,
B*k*Tanh[k + d]/(A^2 + B^2)+c2 == 0}, {k, d, a, b, A, B, c1, c2}]

but Solve will just compute forever and give no result. I've tried also with Reduce and NSolve, but with no luck. I can use approximated answers, so I've tried to use the Taylor series of the second order of the functions Sech Tanh Cos and Sin, but still Solve couldn't give me an answer.


I there something else I could try?


Thank you.



Answer



Notice that most of your transcendal functions are neatly separated into pairs of equations.



This will get you most of the way to your solution


sys={k*Sech[d]*Cos[b]/Sqrt[A^2+B^2]==1, k*Sech[d]*Sin[b]/Sqrt[A^2+B^2]==0, 
A*k*Tanh[d]/(A^2+B^2)+c1==1, B*k*Tanh[d]/(A^2+B^2)+c2==0,
k*Sech[k+d]*Cos[a+b]/Sqrt[A^2+B^2]==1, k*Sech[k+d]*Sin[a+b]/Sqrt[A^2+B^2]==1/10,
A*k*Tanh[k+d]/(A^2+B^2)+c1==11/10, B*k*Tanh[k+d]/(A^2+B^2)+c2==0};
sol=Eliminate[sys, {Sech[d], Tanh[d], Sech[k+d], Tanh[k+d]}]

which instantly tells you


(* B==0 && c2==0 && Cos[a+b]==10 Sin[a+b] && Sin[b]==0 && A!=0 *)


Follow that with


sys /. ToRules[sol] /. True -> Sequence[]

which instantly tells you


(* {(k Cos[b] Sech[d])/Sqrt[A^2] == 1,
(10 k Sech[d+k] Sin[a+b])/Sqrt[A^2]==1,
(k Sech[d+k] Sin[a+b])/Sqrt[A^2]==1/10,
c1+(k Tanh[d])/A==1,
c1+(k Tanh[d+k])/A==11/10}*)


Notice one of those equations is obviously redundant and can be eliminated. Then notice c1 can then be eliminated giving even one fewer equation. Then notice that information from the first step is telling you that b is a multiple of Pi and Cos[b] is either +1 or -1.


If you make use of all this information then you might be able to use Reduce to get to the final solution.


Comments

Popular posts from this blog

front end - keyboard shortcut to invoke Insert new matrix

I frequently need to type in some matrices, and the menu command Insert > Table/Matrix > New... allows matrices with lines drawn between columns and rows, which is very helpful. I would like to make a keyboard shortcut for it, but cannot find the relevant frontend token command (4209405) for it. Since the FullForm[] and InputForm[] of matrices with lines drawn between rows and columns is the same as those without lines, it's hard to do this via 3rd party system-wide text expanders (e.g. autohotkey or atext on mac). How does one assign a keyboard shortcut for the menu item Insert > Table/Matrix > New... , preferably using only mathematica? Thanks! Answer In the MenuSetup.tr (for linux located in the $InstallationDirectory/SystemFiles/FrontEnd/TextResources/X/ directory), I changed the line MenuItem["&New...", "CreateGridBoxDialog"] to read MenuItem["&New...", "CreateGridBoxDialog", MenuKey["m", Modifiers-...

How to thread a list

I have data in format data = {{a1, a2}, {b1, b2}, {c1, c2}, {d1, d2}} Tableform: I want to thread it to : tdata = {{{a1, b1}, {a2, b2}}, {{a1, c1}, {a2, c2}}, {{a1, d1}, {a2, d2}}} Tableform: And I would like to do better then pseudofunction[n_] := Transpose[{data2[[1]], data2[[n]]}]; SetAttributes[pseudofunction, Listable]; Range[2, 4] // pseudofunction Here is my benchmark data, where data3 is normal sample of real data. data3 = Drop[ExcelWorkBook[[Column1 ;; Column4]], None, 1]; data2 = {a #, b #, c #, d #} & /@ Range[1, 10^5]; data = RandomReal[{0, 1}, {10^6, 4}]; Here is my benchmark code kptnw[list_] := Transpose[{Table[First@#, {Length@# - 1}], Rest@#}, {3, 1, 2}] &@list kptnw2[list_] := Transpose[{ConstantArray[First@#, Length@# - 1], Rest@#}, {3, 1, 2}] &@list OleksandrR[list_] := Flatten[Outer[List, List@First[list], Rest[list], 1], {{2}, {1, 4}}] paradox2[list_] := Partition[Riffle[list[[1]], #], 2] & /@ Drop[list, 1] RM[list_] := FoldList[Transpose[{First@li...

mathematical optimization - Minimizing using indices, error: Part::pkspec1: The expression cannot be used as a part specification

I want to use Minimize where the variables to minimize are indices pointing into an array. Here a MWE that hopefully shows what my problem is. vars = u@# & /@ Range[3]; cons = Flatten@ { Table[(u[j] != #) & /@ vars[[j + 1 ;; -1]], {j, 1, 3 - 1}], 1 vec1 = {1, 2, 3}; vec2 = {1, 2, 3}; Minimize[{Total@((vec1[[#]] - vec2[[u[#]]])^2 & /@ Range[1, 3]), cons}, vars, Integers] The error I get: Part::pkspec1: The expression u[1] cannot be used as a part specification. >> Answer Ok, it seems that one can get around Mathematica trying to evaluate vec2[[u[1]]] too early by using the function Indexed[vec2,u[1]] . The working MWE would then look like the following: vars = u@# & /@ Range[3]; cons = Flatten@{ Table[(u[j] != #) & /@ vars[[j + 1 ;; -1]], {j, 1, 3 - 1}], 1 vec1 = {1, 2, 3}; vec2 = {1, 2, 3}; NMinimize[ {Total@((vec1[[#]] - Indexed[vec2, u[#]])^2 & /@ R...