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Solve a system of complicated differential eqs?

Posted 7 years ago

Can anyone familiar with the built-in Solve and partial differential equations explain to me how Mathematica is solving this system?

R = (1/(Sqrt[2 \[Pi]] \[Delta] ) Exp[-(r - 1)^2/(2 \[Delta] ^2)]);
\[CapitalPhi] = (1/(Sqrt[2 \[Pi]] \[Delta] )) Exp[-\[Phi]^2/(
     2 \[Delta] ^2)] + (1/(
     Sqrt[2 \[Pi]] \[Delta] )) Exp[-(\[Phi] - 2 \[Pi]/3)^2/(
     2 \[Delta] ^2)] + (1/(
     Sqrt[2 \[Pi]] \[Delta] )) Exp[-(\[Phi] + 2 \[Pi]/3)^2/(
     2 \[Delta] ^2)];


B = R \[CapitalPhi];
V = TransformedField["Polar" -> "Cartesian", B, {r, \[Phi]} -> {x, y}]

Solve[{\!\(
\*SubscriptBox[\(\[PartialD]\), \(x\)]V\) == 0, \!\(
\*SubscriptBox[\(\[PartialD]\), \(x\)]V\) == 0}, {x, y}]

Im trying to investigate the critical points of this potential field, but for some reason Mathematica is taking a VERY long time to calculate the Solve function. The actual potential V, is not a difficult equation to differentiate by hand. I can't imagine it taking me more than a few minutes to do it myself but I'm lazy.

POSTED BY: Devin Luu
2 Replies
Posted 7 years ago

Hi Devin,

Two comments:

1) In your final solve, both derivatives are with respect to x; I think you intend one x and one y, or just Grad[V,{x,y}]=={0,0}.

2) While the derivatives are easy, the resulting equations are nonlinear and transcendental. You may need to assign values to delta and solve the equations numerically.

Best regards, David

POSTED BY: David Keith
Posted 7 years ago

That was a good catch - I didn't even notice it. It 's still is having trouble evaluating the solutions so it must be the transcendental property you mentioned. I wanted to avoid using actual values for my parameters so that I could generalize the problem but it looks like I will have to go that route. Thank you for your reply.

Devin

POSTED BY: Devin Luu
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