# Christian Blatter

ETH Zurich (Switzerland)

math.ethz.ch/~blatter

Age: 79

 2h comment Has the Collatz Conjecture been proven to be unprovable?The author is talking about numbers $n$ whose binary expansion ends in an "unbounded number of ones". 11h answered Check if a point is inside a rectangle (not knowing the coordinates, but knowing distances to vertices) 1d answered Find the absolute maximum and minimum value of $f$ 2d comment Find a function with the property, or prove it doesn't existYour property, as written, doesn't make much sense. What kind of a variable is $a$, and what does $c_1,c_2,\ldots,c_n$ mean? Note that $f$ can only be partially differentiated with respect to its own variables $x_1$, $\ldots$, $x_n$. 2d answered Evaluate $\sin\left(-\frac{\pi}{6} + \frac{1}{2}\arccos\left(\frac{1}{3}\right)\right)$. 2d answered Clarification of notation $\|fw\|$ 2d answered Values of $a$ for which $f(x)=8 a x-a \sin6x -7x - \sin 5x$ increases 2d comment a linear differential equation with periodic coefficientsLet $a(x)\equiv1$, $b(x)\equiv0$. 2d answered Does $\frac{nx}{1+n \sin(x)}$ converge uniformly on $[a,\pi/2]$ for all $a \in (0,\pi/2]$? 2d revised Difficult Coordinate Geometry and Calculus Questionadded 137 characters in body May 26 answered Difficult Coordinate Geometry and Calculus Question May 25 comment What do $a_0$ ,$a_m$ and $b_m$ terms mean in the Fourier series formula?If $\omega$ is a constant then the Fourier expansion of your combined signal is just $$x(t)=2\sin(\omega t)+\sin(4\omega t)+\cos(\omega t)\ .$$ The whole machinery is only needed when the signal is just an old periodic function, and is not already given as a superposition of specific harmonics. May 25 awarded Investor May 25 comment Probability in a RestaurantWhat is the definition of a "revolving restaurant" and "different seatings" in such a restaurant? May 25 comment What do $a_0$ ,$a_m$ and $b_m$ terms mean in the Fourier series formula?The constant term ${a_0\over2}$ in the Fourier expansion of a periodic function $f$ is the average of $f$ over one period. May 25 comment find all continuous functions $f:\mathbb{R}^n \rightarrow \mathbb{R}$ satisfying $f(x+y)+f(x-y)=2f(x)+2f(y)$ for all $x,y \in \mathbb{R}^n$@John: I don't know yet, or I would have posted it as a solution. May 25 comment find all continuous functions $f:\mathbb{R}^n \rightarrow \mathbb{R}$ satisfying $f(x+y)+f(x-y)=2f(x)+2f(y)$ for all $x,y \in \mathbb{R}^n$If $(x,y)\mapsto B(x,y)$ is bilinear then $f(x):=B(x,x)$ satisfies the given functional equation. May 24 revised Minimum number of moves required to invert a triangular array of coins?added 29 characters in body May 24 answered What is real $R$ so that every subset of Euclidean space with diameter one is inside a ball of radius $R$? May 24 revised Minimum number of moves required to invert a triangular array of coins?added 132 characters in body