# Solving a System of four equations (with logs) [closed]

I am trying to solve a system of four equations in four variables. I have read a number of threads on similar issues and tried to follow the suggestions. But I think it is a bit messy because of the logs and cross products here. This is the exact system:

7*w = (7*w+5*x+2*y+z) * ( 0.76 + 0.12*Log[w] - 0.08*Log[x] - 0.03*Log[y] -
0.07*Log[7*w+5*x + 2*y + z]),
5*x = (7*w+5*x+2*y+z) * ( 0.84 - 0.08*Log[w] + 0.11*Log[x] - 0.02*Log[y] -
0.08*Log[7*w+5*x + 2*y + z]),
2*y = (7*w+5*x+2*y+z) * (-0.45 - 0.03*Log[w] - 0.02*Log[x] + 0.05*Log[y] +
0.12*Log[7*w+5*x + 2*y + z]),
1*z = (7*w+5*x+2*y+z)*(-0.16 + 0*Log[w]- 0*Log[x] - 0*Log[y] + 0.03*Log[7*w+5*x + 2*y + z])


This is an extension of a consumer demand system and we, theoretically, know that there exists a unique solution to this system that is positive.

Trys

• Solve & NSolve : As there should be a solution I tried these, but neither works. I guess that the system has too many logs to handle.
• FindRoot : I started with an initial value of (14,15,10,100) which I get from my data. FindRoot returns the last value (which does not satisfy my system) and the following message.

FindRoot::lstol: The line search decreased the step size to within tolerance specified by AccuracyGoal and PrecisionGoal but was unable..... 

I tried different initial values, including the value returned by FindRoot. I tried to analyze the pattern of the solution value at each step. I didn’t see any pattern, but noticed that the z values become negative early in the process. So, I put bounds on the values. This just stops the code at the minimum value of 0.1. I also tried an exponential system instead of log - same issues.

Reap[FindRoot[{
7*w == (7*w+5*x + 2*y + z)*(0.76 + 0.12*Log[w] - 0.08*Log[x] - 0.03*Log[y] -
0.07*Log[7*w+5*x + 2*y + z]),
5*x == (7*w+5*x + 2*y + z)*(0.84 - 0.08*Log[w] + 0.11*Log[x] - 0.02*Log[y] -
0.08*Log[7*w+5*x + 2*y + z]),
2*y == (7*w + 5*x + 2*y + z)*(-0.45 - 0.03*Log[w] - 0.02*Log[x] + 0.05*Log[y] +
0.12*Log[7*w + 5*x + 2*y + z]),
z == (7*w + 5*x + 2*y + z)*(-0.16 + 0*Log[w] -0*Log[x] -0*Log[y] +
0.03*Log[7*w + 5*x + 2*y + z])},
{{w, 14, 0.1, 500},{x, 15, 0.1, 500},{y, 10, 0.1, 500}, {z, 100, 0.1, 500}},
EvaluationMonitor :> Sow[{w, x, y, z}] ]]

• FindMinimum : As we can write this problem as a minimization problem, I tried this (following the suggestion here). The value returned did not converge the system or equations to zero. I tried with only the first two equations, and that sort of converged to zero.

{g1,g2,g3, g4} = {
7*w - (7*w+5*x+2*y+z)* (0.76+0.12*Log[w]-0.08*Log[x]-0.03*Log[y] -
0.07*Log[7*w + 5*x + 2*y + z]),
5*x - (7*w +5*x+2*y+z)*(0.84-0.08*Log[w]+0.11*Log[x]-0.02*Log[y] -
0.08*Log[7*w + 5*x + 2*y + z]),
2*y - (7*w+5*x+2*y+z)*(-0.45-0.03*Log[w]- 0.02*Log[x] + 0.05*Log[y] +
0.12*Log[7*w+5*x+2*y+z]),
1*z - (7*w+5*x+2*y+z)*(-0.16+0*Log[w]-0*Log[x] - 0*Log[y] +
0.03*Log[7*w+5*x+2*y+z])};

subdomain = 0 < w < 100 && 0 < x < 100 && 0 < y < 100 && 0 < z < 100;
res = FindMinimum[{Total[{g1,g2,g3,g4}^2],subdomain},{w,x,y,z}, AccuracyGoal->5]
{g1,g2,g3,g4}/.res[[2]]
`

Hope this is engaging enough for the experts here! Any ideas how I should find the solution or why can’t I? It’s the first time I am using Mathematica, and unfortunately the first time I am empirically solving a system/optimizing! Thanks a lot.

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## closed as off-topic by Michael E2, Yves Klett, ciao, Sjoerd C. de Vries, ÖskåJul 1 '14 at 9:55

This question appears to be off-topic. The users who voted to close gave this specific reason:

• "This question arises due to a simple mistake such as a trivial syntax error, incorrect capitalization, spelling mistake, or other typographical error and is unlikely to help any future visitors, or else it is easily found in the documentation." – Michael E2, Yves Klett, ciao, Sjoerd C. de Vries, Öskå
If this question can be reworded to fit the rules in the help center, please edit the question.

@Sektor Thanks for the edits. I promise to get better at this:). –  Divergent-Economist Mar 18 '14 at 10:16
Oh, no need to thank me .. I hope I preserved the consistency :) Have a nice day ! –  Sektor Mar 18 '14 at 10:17
Your system simply has no solutions. What makes you so sure it does? –  george2079 Mar 19 '14 at 20:25
This is a system of 4 goods with prices (w,x,y,z). We equate the expenditure of buying (7,5,2,1) units to the total expenditure*budget shares. The budget shares come from a demand system, and we have a theorem which proves that there exists a price vector solving this. Hence, I thought that I might not be using the best methods/code to solve this. But now I am re-checking my model too. Thanks. –  Divergent-Economist Mar 19 '14 at 21:31
Your procedure looks basically correct -- first thing I'd advise is work through a problem with just two 'goods' assuming that makes sense. –  george2079 Mar 19 '14 at 21:56