Christian Blatter

ETH Zurich (Switzerland)

math.ethz.ch/~blatter

Age: 79

11h
answered Why is $\partial_z\partial_{\bar z}=\frac14\left(\partial_r^2+\frac1r\partial_r+\frac1{r^2}\partial_{\theta}^2\right)$?
11h
answered Geometrical construction for Snell's law?
12h
answered Starting either Advanced Calculus or Introductory Analysis
14h
comment Writing a complex function $f(x,y)=f(x+iy)$ as function of complex variable $z$.
What do you mean by "differentiable complex function"? The function $f(x,y):=x-iy$ is complex-valued and ${\mathbb R}$-differentiable, but not ${\mathbb C}$-differentiable.
17h
comment How to prove that the lines in a polygonal approximation of a simple closed curve do not intersect as n gets large
You have assumed $\phi\in C^1$ (which was not mentioned in the question). Consider two congruent logarithmic spirals spiraling towards the origin and concatenated there, and the outer ends joined somehow. (Maybe this is not a counterexample; it just shows that things can become tricky without the $C^1$ assumption.)
20h
revised Advice needed for a problem from Advanced Calculus by Patrick M. Fitzpatrick
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1d
answered Advice needed for a problem from Advanced Calculus by Patrick M. Fitzpatrick
1d
comment Intuition behind the substitution method of integration
It's depressing to see this question closed. I'd like to see some interesting thoughts on it by the people who voted to close. When the question was here over three years ago the answers just regurgitated the proof of the substitution rule, but didn't address the real issue of the question.
1d
answered The domain onto or into or neither?
1d
answered Barycentric coordinates in a triangle - proof
1d
answered How do I see that the complex ODE $z^{''} - 2iz = 0$ has a $2$-dimensional solution space?
1d
revised Is the standard scalar product in a coordinate space basis independent?
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1d
comment How do I see that the complex ODE $z^{''} - 2iz = 0$ has a $2$-dimensional solution space?
Any IVP $z''-2i z=0$, $z(t_0)=z_0$, $z'(t_0)=w_0$ can be solved in terms of the solutions you have found.
1d
comment How can I prove that $\partial\varphi\neq0\Rightarrow\bar\partial\partial\varphi>0$?
Would $\bar\partial\partial\phi\geq0$ suffice? This is easy to show.
2d
answered What is the gradient of a function?
2d
revised Show that Mobius transformation $S$ commute with $T$ if $S$ and $T$ have the same fixed point.
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2d
answered Probability of at least one random number out of 3 being greater than 3 other random numbers?
2d
answered Functional equation $ f(x)+f(x+1)=x$
2d
revised simple formal proof $ \lfloor (z+1)/c \rfloor \lt z $ for $ c \ge 2 $
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2d
answered simple formal proof $ \lfloor (z+1)/c \rfloor \lt z $ for $ c \ge 2 $
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