How to get the Lagrangian that generates certain diff equation ?

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The discussion focuses on obtaining the Lagrangian that generates a specific second-order ordinary differential equation, particularly through the lens of the inverse problem of variational calculus. Key references include N. Akhiezer's "The Calculus of Variations" (1962) and R. Santilli's "Foundations of Theoretical Mechanics" (1978, 1983), which provide practical examples and methodologies for deriving the Lagrangian. The discussion emphasizes the need for resources that avoid general theorems and geometrical complexities, targeting one degree of freedom.

PREREQUISITES
  • Understanding of ordinary differential equations (ODEs)
  • Familiarity with the Euler-Lagrange equation
  • Knowledge of variational calculus principles
  • Basic concepts of Hamiltonian mechanics and Legendre transformations
NEXT STEPS
  • Study N. Akhiezer's "The Calculus of Variations" for practical examples of one-dimensional problems
  • Review R. Santilli's "Foundations of Theoretical Mechanics II" for insights on Kobussen's method
  • Explore the Douglas approach in R. Santilli's "Foundations of Theoretical Mechanics I" for n-dimensional applications
  • Learn about the inverse Legendre transformation to derive Lagrangians from Hamiltonians
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Researchers, physicists, and mathematicians interested in variational calculus, particularly those focused on deriving Lagrangians for specific differential equations in a one-dimensional context.

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how to get the Lagrangian that generates certain diff equation ?

I have an ordinary differential equation of second order. I am looking for the Lagrangian(s) for which this equation is the Euler-Lagrange equation. I need a practical reference specifically written for one degree of freedom (one equation), I'm not interested in general theorems of existence or highly geometrical language. I found some reference by Darboux in 1894 but its in French. Any english reference?
 
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Got it. The problem is known as 'inverse problem of variational calculus'. For future references:

N. Akhiezer, "The Calculus of Variations", 1962:

page 166: example of 1D problem

R. Santilli, "Foundations of Theoretical Mechanics II", Springer-Verlag, 1983:

page 353: reworking the same example as Akhiezer
page 315: by knowing one first integral, Kobussen's method

R. Santilli, "Foundations of Theoretical Mechanics I", Springer-Verlag, 1978:

page 201: Douglas approach for n-dimension, easily applied to n=1
page 208: first obtaining Hamiltonian and then get the Lagrangian through inverse Legendre transformation
 
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smallphi said:
Got it. The problem is known as 'inverse problem of variational calculus'. For future references:

N. Akhiezer, "The Calculus of Variations", 1962:

page 166: example of 1D problem

R. Santilli, "Foundations of Theoretical Mechanics II", Springer-Verlag, 1983:

page 353: reworking the same example as Akhiezer
page 315: by knowing one first integral, Kobussen's method

R. Santilli, "Foundations of Theoretical Mechanics I", Springer-Verlag, 1978:

page 201: Douglas approach for n-dimension, easily applied to n=1
page 208: first obtaining Hamiltonian and then get the Lagrangian through inverse Legendre transformation

Thanks for the pointers!
 

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