Euler Lagrange equation and a varying Lagrangian

In summary: Y)} \cdot \nabla_\mu \nabla_\nu \right) X.\end{align}This last term is the variation with respect to $X$ and is given by\begin{align}\frac{\delta^2 L}{\delta X \delta Y} &= \frac{\partial^2 L}{\partial X} - \frac{1}{2!}\left(\frac{\partial L}{\partial (\nabla_\mu X)} \cdot \nabla_\mu \right) Y - \frac{1}{2!}\left(\frac{\partial L}{\partial (\
  • #1
Tamin Ayoub
1
0
Homework Statement
The variation of the Lagrangian with respect to PSI and X
Relevant Equations
Euler Lagrange equation
Hello, I have been working on the three-dimensional topological massive gravity (I'm new to this field) and I already faced the first problem concerning the mathematics, after deriving the lagrangian from the action I had a problem in variating it
Here is the Lagrangian
Sans titre.png


The first variation should be with respect to Ψ I tried to find this result
22.png

but I had a sign different, my result had a Minus sign.

The second variation is with respect to the vector X, note that
55.png

and the result is
1593770698677.png

I tried to use the euler lagrange equation to find this result but i don't have much experience dealing with this wedge product variation

thank you in advanced
 
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  • #2
for helping me The Euler-Lagrange equation for the variation with respect to $\Psi$ is given by\begin{align}\frac{\delta L}{\delta \Psi} = \frac{\partial L}{\partial \Psi} - \nabla_\mu \frac{\partial L}{\partial (\nabla_\mu \Psi)} = 0.\end{align}A few notes:1) The second term on the right-hand side, $\nabla_\mu \frac{\partial L}{\partial (\nabla_\mu \Psi)}$, involves taking the partial derivative of the Lagrangian with respect to the covariant derivative of $\Psi$. This is done using the Leibniz rule.2) The wedge product of two vectors, $X$ and $Y$, is defined as\begin{align}X \wedge Y = \frac{1}{2!} (X \cdot \nabla) Y - \frac{1}{2!} (Y \cdot \nabla) X.\end{align}Using this definition, we can rewrite the second variation in terms of derivatives of the Lagrangian with respect to $X$ and $Y$:\begin{align}\frac{\delta^2 L}{\delta X \delta Y} &= \frac{\partial^2 L}{\partial X \partial Y} - \frac{1}{2!}\left(\frac{\partial L}{\partial (\nabla_\mu X)} \cdot \nabla_\mu \right) Y - \frac{1}{2!}\left(\frac{\partial L}{\partial (\nabla_\mu Y)} \cdot \nabla_\mu \right) X \\&+ \frac{1}{4!}\left(\frac{\partial L}{\partial (\nabla_\mu \nabla_\nu X)} \cdot \nabla_\mu \nabla_\nu \right) Y - \frac{1}{4!}\left(\frac{\partial L}{\partial (\nabla_\mu
 

1. What is the Euler-Lagrange equation?

The Euler-Lagrange equation is a mathematical equation that is used to find the function that minimizes or maximizes a given functional. It is commonly used in the field of calculus of variations and is named after the mathematicians Leonhard Euler and Joseph-Louis Lagrange.

2. How is the Euler-Lagrange equation derived?

The Euler-Lagrange equation is derived by applying the calculus of variations to a functional, which is a mathematical expression that takes in a function as its input and outputs a real number. By setting the first variation of the functional to zero, the Euler-Lagrange equation can be obtained.

3. What is a varying Lagrangian?

A varying Lagrangian is a Lagrangian that depends on both the independent variable and the dependent variable. This means that the Lagrangian is not constant and can change as the system evolves over time. This is in contrast to a constant Lagrangian, which does not change with time.

4. How does a varying Lagrangian affect the Euler-Lagrange equation?

A varying Lagrangian can affect the Euler-Lagrange equation by introducing additional terms that must be considered in the derivation. This can make the equation more complex and may require more advanced mathematical techniques to solve.

5. What are the applications of the Euler-Lagrange equation and varying Lagrangians?

The Euler-Lagrange equation and varying Lagrangians have many applications in physics, engineering, and mathematics. They are commonly used in the study of classical mechanics, optimal control theory, and partial differential equations. They are also used in fields such as computer graphics, robotics, and economics.

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