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Real-valued functions of real variable, analytic properties of functions and sequences, limits, continuity, smoothness of these.

5 votes
3 answers
158 views

Reference for a Grünwald–Letnikov-type definition of the $n$-th derivative of a function

Let $U\subset\mathbb R$ be an open set. Let $n\in\mathbb N$ and suppose that $f\in\mathcal C^n(U)$, i.e. that $f$ is $n$-times continuously differentiable on $U$. The $n$-th derivative of $f$, denoted …
Maximilian Janisch's user avatar
6 votes
Accepted

Can I find a bump function $\psi$ such that $\nabla\log\psi$ vanishes too?

Elaborating the comment by Wojowu: If we take a look at $n=1$ and $\psi\in C^\infty_{\text c}(\mathbb R)$ is a function satisfying conditions 1., 2. and 3. of your question, then for every $x\in]-1,1[ …
Maximilian Janisch's user avatar
3 votes
0 answers
450 views

Opposite of the curl operator and Biot-Savart kernel

Note: I just realized that using $\omega$ and $w$ might not have been the smartest choice of notation -- Sorry about that. Let $\renewcommand{\div}{\operatorname{\div}}Q_0, Q_1$ be two real numbers, $ …
Maximilian Janisch's user avatar
14 votes
3 answers
2k views

How to prove $e^x\left|\int_x^{x+1}\sin(e^t) \,\mathrm d t\right|\le 1.4$?

Related question asked by me on Math SE a few days ago: How to prove $e^x\left|\int_x^{x+1}\sin(e^t) \,\mathrm d t\right|\le 1.4$? A few days ago, somebody asked How to prove $ \mathrm{e}^x\left|\int …
Maximilian Janisch's user avatar