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

2 votes
2 answers
177 views

Upper/Lower bounds of real-analytic functions with infinite Taylor series

For example, in 1-D, given some positive increasing polynomial $p(x) = a_1x+\ldots+a_nx^n$, $p(0) = 0$, there exists constants $b_1,b_2$ such that for $x<\delta$, for some $\delta > 0$, we have that $ …
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0 votes
0 answers
48 views

First nonzero derivative bounded below (2 dimensions)

Let $B\subseteq \Bbb{R}^2$ be a closed ball of radius $\delta < 1$ centered at $(0,0)$. Let $f:B\to \Bbb{R}_{\geq 0}$ be real-analytic, have only one zero (at $(0,0)$) and be strictly increasing along …
Doofenshmert's user avatar
0 votes
0 answers
28 views

$ \sup_{\theta \in [0,2\pi)}\max_{r\leq \delta}\frac{\log\left(\frac{f(r,\theta)}{f(\delta,\...

$\textbf{Conjecture.}$ Let $B\subseteq \Bbb{R}^2$ be a closed ball centered on $(0,0)$ of radius $\delta <1$. Let $f:B\to \Bbb{R}_{\geq 0}$ be real analytic and suppose that $(0,0)$ is the only zero o …
Doofenshmert's user avatar
0 votes
0 answers
70 views

Minimum Slice of Real Analytic Function in Two Variables

Let $B\subseteq \Bbb{R}^2$ be a closed ball of radius $\delta < 1$ centered at $(0,0)$. Let $f:B\to \Bbb{R}_{\geq 0}$ be real analytic and have only one zero, namely $(0,0)$. Moreover, assume that $f$ …
Doofenshmert's user avatar
2 votes
0 answers
207 views

Seeking alternative elementary proof instead of applying Lojaseiwicz's inequality for $f(x,y...

Let $B\subseteq \Bbb{R}^2$ be a closed ball centered on $(0,0)$ of radius $0<\delta<1$. Let $f:B\to \Bbb{R}_{\geq 0}$ be real analytic and contain only one zero in $A$, namely $(0,0)$. In other words, …
Doofenshmert's user avatar
1 vote
1 answer
74 views

Lower bound of $\frac{f(x)}{x^{n+1}}$

Let $f:[0,a]\to \Bbb{R}_{\geq 0}$ be real analytic, $a<1$. Furthermore, $f(0) = 0$ and $f$ is strictly increasing on $[0,a]$. Let $n\in \Bbb{N}$ be the smallest positive integer such that $f^{(n)}(0)\ …
Doofenshmert's user avatar
0 votes
0 answers
51 views

Vectors of complex exponentials span $\mathbf{C}^N$

Let $Q = [0,1]\times [0,1]$. Let $1\leq k < \infty$ and $\{(x_l,\xi_l)\}_{l=1}^{k}\subseteq Q$ be such that $(x_i,\xi_i)\ne (x_j, \xi_j)$ for $i\ne j$. Additionally, for $1\leq l \leq k$, let $n_l\in …
Doofenshmert's user avatar
1 vote
1 answer
124 views

approximating differentiable functions with double trigonometric polynomials

Let $Q = [0,1]^2$. For sake of notation, let $$ f^{(i,j)}(x,\xi) = \frac{\partial^{i+j}}{\partial x^i \partial \xi^j}f(x,\xi). $$ Fix some non-negative integer $k$. Moreover let $f\in C^k(Q)$ if $$ \| …
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