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Valent
  • Member for 10 years, 5 months
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6 votes
3 answers
2k views

Let $f:\mathbb{R}^2\to\mathbb{R}$ be a $C^1$ function. Prove that the restriction is not injective.

6 votes
1 answer
86 views

Prove that $\exists k\in \mathbb{N}^*$ such that $\|a-a_k\|<\varepsilon$

4 votes
4 answers
724 views

How can I solve this O.D.E.?

4 votes
2 answers
103 views

Finding $\iint_S \nabla \times F\ dS$

4 votes
2 answers
287 views

Is this a group or a vector space?

3 votes
1 answer
42 views

Determine a volume on the first octant using triple integrals

3 votes
2 answers
123 views

Prove that $f(x)=\frac{1}{x^2+1}$ is uniformly continuous in $\mathbb{R}$

3 votes
2 answers
88 views

Prove that some function is the solution of some equation

3 votes
1 answer
81 views

How can I prove this proposition that seems obvious?

3 votes
2 answers
187 views

Is true that $n=m$?

3 votes
1 answer
78 views

Prove that $Df(p)=f(p)T$ where $T(q)=\int_{0}^1q$

3 votes
0 answers
240 views

$\int_{-\infty}^{+\infty}f \ d\phi$ exists if $f(x)$ is continuous, $\phi$ of bounded variation

3 votes
1 answer
127 views

$f(z)=\sum_{k=0}^\infty a_kz^{k}$ with $\sum |a_k|<\infty$ is of bounded variation on circle $|z|=1$

3 votes
1 answer
144 views

Triangle inequality infinite terms $|a-b|\leq \sum_{i=1}^\infty|x_{i}-x_{i-1}|$

3 votes
2 answers
33 views

Good definition for $\int_a^\infty X(t)dt$, where $X(t)\in M_n$

3 votes
1 answer
91 views

If $D=\{(x,y):x^2+y^2\leq 1\}$. Show there is $p_0\in D$ such that $T(p_0)=(0,0)$

3 votes
0 answers
39 views

$\|A\|\leq \sum_{m=0}^\infty \|A_m\|$

3 votes
1 answer
207 views

Prove that there is a continuous $f$ not Riemann-Stieltjes integrable with respect to $\phi$

3 votes
1 answer
47 views

Find the Laplace $\mathcal{L}(y(t))$ if $y'(t)-y(t)+\int_0^t y(u)e^{\alpha(t-u)}du=\alpha e^{\alpha t}$

3 votes
1 answer
48 views

Converts into $\frac{\partial^2 W}{\partial u^2}+\frac{\partial^2 W}{\partial v^2}=0$

3 votes
1 answer
344 views

Is $Y_n=S_n^3-3nS_n$ a martingale?

3 votes
1 answer
321 views

$e^{X_t}$ is martingale using Ito's Formula

2 votes
1 answer
80 views

$\int_0^{\infty} e^{-x}\cos xdx\approx \frac{h}{2}\frac{\sinh(h)}{\cosh(h)-\cos h} $

2 votes
1 answer
52 views

$\sup_{\Gamma}\sum |\phi(x_i)-\phi(x_{i-1})|=\infty$ but $\lim_{|\Gamma|\to 0}\sum (\phi(x_i)-\phi(x_{i-1}))$ exists

2 votes
0 answers
22 views

Implicit theorem in $\mathbb{R}^3$ there is $\xi:(t_0-\varepsilon,t_0+\varepsilon)\to\mathbb{R}$ such that $F(t,\xi(t),\xi'(t))=0$

2 votes
2 answers
106 views

Simple question on Mean value theorem

2 votes
1 answer
55 views

How to prove that if $S(S,f,\{p_i\})=\sum f(p_j)A(D_j)$ then $|\iint_{R}f-S|(S,f,\{p_i\})|<\varepsilon$

2 votes
1 answer
88 views

If $g(x,y)=(x+f(y),y+f(x))$ Why this function $g$ is onto?

2 votes
0 answers
126 views

How to prove that $\|x+y\|_p<\|x\|_p+\|y\|_p$? [duplicate]

2 votes
0 answers
44 views

How to prove that: $\exists C>1$ such that $C||g(y)||\geq ||y||$?