Non-linear system of 3 equations











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I have a system of 3 equations with the unknown $(x,y,z)$ to solve in the space $mathbb{R}^2 times mathbb{R}_+^{*}$ :
$$m=x-y \ v=x^2(z-1) \gamma=x^3(z^3+3z-2)$$



with $m in mathbb{R}$, $v>0$ , $gamma in mathbb{R}$ and $z>1$



Numerically, I easily find the solution with a solver, but I cannot figure out whether a closed-form formula of the solution exists.



Thanks.










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  • 1




    It looks like you can treat the two equations in $x,z$ as a separate pair and solve those, then finish by solving the first. Have you attempted to factor $z^3+3z-2$?
    – abiessu
    Nov 21 at 15:08






  • 1




    Actually, $z>1$, I tried but I end up solving $$gamma=(z^3+3z-2)*(frac{v}{z-1})^{frac{3}{2}}$$
    – Canardini
    Nov 21 at 15:31

















up vote
0
down vote

favorite












I have a system of 3 equations with the unknown $(x,y,z)$ to solve in the space $mathbb{R}^2 times mathbb{R}_+^{*}$ :
$$m=x-y \ v=x^2(z-1) \gamma=x^3(z^3+3z-2)$$



with $m in mathbb{R}$, $v>0$ , $gamma in mathbb{R}$ and $z>1$



Numerically, I easily find the solution with a solver, but I cannot figure out whether a closed-form formula of the solution exists.



Thanks.










share|cite|improve this question




















  • 1




    It looks like you can treat the two equations in $x,z$ as a separate pair and solve those, then finish by solving the first. Have you attempted to factor $z^3+3z-2$?
    – abiessu
    Nov 21 at 15:08






  • 1




    Actually, $z>1$, I tried but I end up solving $$gamma=(z^3+3z-2)*(frac{v}{z-1})^{frac{3}{2}}$$
    – Canardini
    Nov 21 at 15:31















up vote
0
down vote

favorite









up vote
0
down vote

favorite











I have a system of 3 equations with the unknown $(x,y,z)$ to solve in the space $mathbb{R}^2 times mathbb{R}_+^{*}$ :
$$m=x-y \ v=x^2(z-1) \gamma=x^3(z^3+3z-2)$$



with $m in mathbb{R}$, $v>0$ , $gamma in mathbb{R}$ and $z>1$



Numerically, I easily find the solution with a solver, but I cannot figure out whether a closed-form formula of the solution exists.



Thanks.










share|cite|improve this question















I have a system of 3 equations with the unknown $(x,y,z)$ to solve in the space $mathbb{R}^2 times mathbb{R}_+^{*}$ :
$$m=x-y \ v=x^2(z-1) \gamma=x^3(z^3+3z-2)$$



with $m in mathbb{R}$, $v>0$ , $gamma in mathbb{R}$ and $z>1$



Numerically, I easily find the solution with a solver, but I cannot figure out whether a closed-form formula of the solution exists.



Thanks.







real-analysis linear-algebra systems-of-equations






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edited Nov 21 at 15:30

























asked Nov 21 at 15:01









Canardini

2,4651519




2,4651519








  • 1




    It looks like you can treat the two equations in $x,z$ as a separate pair and solve those, then finish by solving the first. Have you attempted to factor $z^3+3z-2$?
    – abiessu
    Nov 21 at 15:08






  • 1




    Actually, $z>1$, I tried but I end up solving $$gamma=(z^3+3z-2)*(frac{v}{z-1})^{frac{3}{2}}$$
    – Canardini
    Nov 21 at 15:31
















  • 1




    It looks like you can treat the two equations in $x,z$ as a separate pair and solve those, then finish by solving the first. Have you attempted to factor $z^3+3z-2$?
    – abiessu
    Nov 21 at 15:08






  • 1




    Actually, $z>1$, I tried but I end up solving $$gamma=(z^3+3z-2)*(frac{v}{z-1})^{frac{3}{2}}$$
    – Canardini
    Nov 21 at 15:31










1




1




It looks like you can treat the two equations in $x,z$ as a separate pair and solve those, then finish by solving the first. Have you attempted to factor $z^3+3z-2$?
– abiessu
Nov 21 at 15:08




It looks like you can treat the two equations in $x,z$ as a separate pair and solve those, then finish by solving the first. Have you attempted to factor $z^3+3z-2$?
– abiessu
Nov 21 at 15:08




1




1




Actually, $z>1$, I tried but I end up solving $$gamma=(z^3+3z-2)*(frac{v}{z-1})^{frac{3}{2}}$$
– Canardini
Nov 21 at 15:31






Actually, $z>1$, I tried but I end up solving $$gamma=(z^3+3z-2)*(frac{v}{z-1})^{frac{3}{2}}$$
– Canardini
Nov 21 at 15:31












1 Answer
1






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oldest

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1
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A standard way to solve the last two equations, would be to square the last and cube the other to solve for $z$ (expect when $x=0$, but in this case $y=-m$ and $z$ can be anything):
$$
v³(z³+3z-2)²-gamma^2(z-1)³ = 0
$$

This gives a sixth order polynomial that you still have to check for extraneous solutions (as we have squared and lost the sign of $gamma$ and $x$). But than, using the second, we get:
$$
x = pmsqrt{frac{v}{z-1}}
$$

Which also requires $z>1$ for a solution, and finally using the first one:
$$
y = x - m
$$






share|cite|improve this answer





















  • Yes, exactly, but how do I solve the first equation ? I feel anyhow I need to you use a numerical solver .
    – Canardini
    Nov 21 at 15:35












  • It is not possible if you cannot reduce the degree of the polynomial with some cleaver trick, which does not seems to be possible as your coefficients depends on the parameters.
    – Eduardo Elael
    Nov 21 at 15:50











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1 Answer
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up vote
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A standard way to solve the last two equations, would be to square the last and cube the other to solve for $z$ (expect when $x=0$, but in this case $y=-m$ and $z$ can be anything):
$$
v³(z³+3z-2)²-gamma^2(z-1)³ = 0
$$

This gives a sixth order polynomial that you still have to check for extraneous solutions (as we have squared and lost the sign of $gamma$ and $x$). But than, using the second, we get:
$$
x = pmsqrt{frac{v}{z-1}}
$$

Which also requires $z>1$ for a solution, and finally using the first one:
$$
y = x - m
$$






share|cite|improve this answer





















  • Yes, exactly, but how do I solve the first equation ? I feel anyhow I need to you use a numerical solver .
    – Canardini
    Nov 21 at 15:35












  • It is not possible if you cannot reduce the degree of the polynomial with some cleaver trick, which does not seems to be possible as your coefficients depends on the parameters.
    – Eduardo Elael
    Nov 21 at 15:50















up vote
1
down vote













A standard way to solve the last two equations, would be to square the last and cube the other to solve for $z$ (expect when $x=0$, but in this case $y=-m$ and $z$ can be anything):
$$
v³(z³+3z-2)²-gamma^2(z-1)³ = 0
$$

This gives a sixth order polynomial that you still have to check for extraneous solutions (as we have squared and lost the sign of $gamma$ and $x$). But than, using the second, we get:
$$
x = pmsqrt{frac{v}{z-1}}
$$

Which also requires $z>1$ for a solution, and finally using the first one:
$$
y = x - m
$$






share|cite|improve this answer





















  • Yes, exactly, but how do I solve the first equation ? I feel anyhow I need to you use a numerical solver .
    – Canardini
    Nov 21 at 15:35












  • It is not possible if you cannot reduce the degree of the polynomial with some cleaver trick, which does not seems to be possible as your coefficients depends on the parameters.
    – Eduardo Elael
    Nov 21 at 15:50













up vote
1
down vote










up vote
1
down vote









A standard way to solve the last two equations, would be to square the last and cube the other to solve for $z$ (expect when $x=0$, but in this case $y=-m$ and $z$ can be anything):
$$
v³(z³+3z-2)²-gamma^2(z-1)³ = 0
$$

This gives a sixth order polynomial that you still have to check for extraneous solutions (as we have squared and lost the sign of $gamma$ and $x$). But than, using the second, we get:
$$
x = pmsqrt{frac{v}{z-1}}
$$

Which also requires $z>1$ for a solution, and finally using the first one:
$$
y = x - m
$$






share|cite|improve this answer












A standard way to solve the last two equations, would be to square the last and cube the other to solve for $z$ (expect when $x=0$, but in this case $y=-m$ and $z$ can be anything):
$$
v³(z³+3z-2)²-gamma^2(z-1)³ = 0
$$

This gives a sixth order polynomial that you still have to check for extraneous solutions (as we have squared and lost the sign of $gamma$ and $x$). But than, using the second, we get:
$$
x = pmsqrt{frac{v}{z-1}}
$$

Which also requires $z>1$ for a solution, and finally using the first one:
$$
y = x - m
$$







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Nov 21 at 15:32









Eduardo Elael

1865




1865












  • Yes, exactly, but how do I solve the first equation ? I feel anyhow I need to you use a numerical solver .
    – Canardini
    Nov 21 at 15:35












  • It is not possible if you cannot reduce the degree of the polynomial with some cleaver trick, which does not seems to be possible as your coefficients depends on the parameters.
    – Eduardo Elael
    Nov 21 at 15:50


















  • Yes, exactly, but how do I solve the first equation ? I feel anyhow I need to you use a numerical solver .
    – Canardini
    Nov 21 at 15:35












  • It is not possible if you cannot reduce the degree of the polynomial with some cleaver trick, which does not seems to be possible as your coefficients depends on the parameters.
    – Eduardo Elael
    Nov 21 at 15:50
















Yes, exactly, but how do I solve the first equation ? I feel anyhow I need to you use a numerical solver .
– Canardini
Nov 21 at 15:35






Yes, exactly, but how do I solve the first equation ? I feel anyhow I need to you use a numerical solver .
– Canardini
Nov 21 at 15:35














It is not possible if you cannot reduce the degree of the polynomial with some cleaver trick, which does not seems to be possible as your coefficients depends on the parameters.
– Eduardo Elael
Nov 21 at 15:50




It is not possible if you cannot reduce the degree of the polynomial with some cleaver trick, which does not seems to be possible as your coefficients depends on the parameters.
– Eduardo Elael
Nov 21 at 15:50


















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