Integrability of a differential condition

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The discussion centers on the integrability of the differential condition dq_3 = B_1 dq_1 + B_2 dq_2 as presented in C. Lanczos's "The Variational Principles of Mechanics." It establishes that for dq_3 to be integrable, the condition ∂B_1/∂q_2 = ∂B_2/∂q_1 must hold true. This is derived from the equality of mixed partial derivatives, ∂²f/∂q₁∂q₂ = ∂²f/∂q₂∂q₁, which implies that B_1 and B_2 can be expressed as partial derivatives of a function f. The discussion clarifies the relationship between the differential condition and the necessary equality of derivatives.

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hrmello
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I'm reading "The variational principles of mechanics", written by C. Lanczos and he said that, if one have the condition dq_3 = B_1 dq_1 + B_2 dq_2 and one want to know if there is a finite relation between the q_i, on account the given condition, one must have the condition \frac{\partial B_1}{\partial q_2} = \frac{\partial B_2}{\partial q_1}.

What I don't understand is how the condition on the differentials imply the equality between the derivatives above.
 
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The point is, under certain conditions(which are always satisfied in physics), we have \frac{\partial^2 f}{\partial q_1 \partial q_2}=\frac{\partial^2 f}{\partial q_2 \partial q_1}. Now for dq_3 to be integrable, there should be some f such that B_1=\frac{\partial f}{\partial q_1} and B_2=\frac{\partial f}{\partial q_2}. But how can we check that? We can use the equation I mentioned:
\frac{\partial^2 f}{\partial q_1 \partial q_2}=\frac{\partial^2 f}{\partial q_2 \partial q_1} \Rightarrow \frac{\partial B_2}{\partial q_1}=\frac{\partial B_1}{\partial q_2}.
 
Thanks, Shyan! :D
 

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