Not understanding these manipulations involving Partial Derivatives

Click For Summary
The discussion focuses on the process of differentiating a function f with respect to its arguments and then with respect to time t. A clarification is provided by substituting u = tx and v = ty, leading to the expression for the partial derivative of f with respect to t. This method illustrates how to apply the chain rule in the context of partial derivatives. The initial confusion regarding the notation "tx" is resolved through this explanation. Overall, the thread emphasizes the importance of understanding the chain rule in partial differentiation.
MatinSAR
Messages
673
Reaction score
204
Homework Statement
Find partial derivatives
Relevant Equations
dy/dx=(dy/dt)(dt/dx)
Can someone please help me to find out what happened here ?

1675445974557.png
 
Physics news on Phys.org
It's differentiating ##f## with respect to its arguments, then differentiating the arguments with respect to ##t##. It might be clearer if you write ##u = tx## and ##v=ty##, then
$$\partial f(u,v) / \partial t = (\partial f/ \partial u) (\partial u/ \partial t) + (\partial f/ \partial v) (\partial v/ \partial t)$$
 
  • Like
Likes SammyS, Mark44 and MatinSAR
ergospherical said:
It's differentiating ##f## with respect to its arguments, then differentiating the arguments with respect to ##t##. It might be clearer if you write ##u = tx## and ##v=ty##, then
$$\partial f(u,v) / \partial t = (\partial f/ \partial u) (\partial u/ \partial t) + (\partial f/ \partial v) (\partial v/ \partial t)$$
That "tx" confused me ...
Now it's clear...
Thank you for your time 🙏🙏
 
First, I tried to show that ##f_n## converges uniformly on ##[0,2\pi]##, which is true since ##f_n \rightarrow 0## for ##n \rightarrow \infty## and ##\sigma_n=\mathrm{sup}\left| \frac{\sin\left(\frac{n^2}{n+\frac 15}x\right)}{n^{x^2-3x+3}} \right| \leq \frac{1}{|n^{x^2-3x+3}|} \leq \frac{1}{n^{\frac 34}}\rightarrow 0##. I can't use neither Leibnitz's test nor Abel's test. For Dirichlet's test I would need to show, that ##\sin\left(\frac{n^2}{n+\frac 15}x \right)## has partialy bounded sums...