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What functions check these conditions?
TheBlackPuzzle913   2
N Saturday at 10:02 PM by Filipjack
Source: RMO shortlist, Mihai Bunget and Dragoș Gabriel Borugă
Find all functions $ f : \mathbb{R} \rightarrow (0, \infty) $ that are twice differentiable and satisfy $ 3(f'(x))^2 \le 2f(x)f''(x) , \forall x \in \mathbb{R} $
2 replies
TheBlackPuzzle913
Saturday at 8:09 PM
Filipjack
Saturday at 10:02 PM
Show the existence of a neighborhhod
Alidq   0
Mar 28, 2025
Source: some derivative problem handout
Let $f:(a,b) \rightarrow \mathbb{R}$ a twice differentiable function with a continuous second derivative, for which there exists a unique $t \in (a,b)$ such that $f(t)=0$. If $f'(t) \neq 0$, show that there exists a neighborhood $V = (t- \delta, t+ \delta)$ of $t$ such that for any $x_0 \in V$ , the sequence defined by the recurrence relation $$x_{n+1} = x_n - \frac{f(x_n)}{f'(x_n)}$$converges to $t$.
0 replies
Alidq
Mar 28, 2025
0 replies
f"(x)>0, show that int(f(x)cosx dx) >0
Sayan   11
N Mar 28, 2025 by Mathzeus1024
Source: ISI(BS) 2009 #2
Let $f(x)$ be a continuous function, whose first and second derivatives are continuous on $[0,2\pi]$ and $f''(x) \geq 0$ for all $x$ in $[0,2\pi]$. Show that
\[\int_{0}^{2\pi} f(x)\cos x dx \geq 0\]
11 replies
Sayan
May 5, 2012
Mathzeus1024
Mar 28, 2025
Double derivative coated integral
RenheMiResembleRice   1
N Mar 28, 2025 by HacheB2031
Source: Lufang Yue, Lianru Meng
Show that J>0. For this to be true, I guess we also need the condition for c and d to be positive, isn't that right?
1 reply
RenheMiResembleRice
Mar 28, 2025
HacheB2031
Mar 28, 2025
prove that there exists \xi
Peter   21
N Mar 25, 2025 by Alphaamss
Source: IMC 1998 day 1 problem 4
The function $f: \mathbb{R}\rightarrow\mathbb{R}$ is twice differentiable and satisfies $f(0)=2,f'(0)=-2,f(1)=1$.
Prove that there is a $\xi \in ]0,1[$ for which we have $f(\xi)\cdot f'(\xi)+f''(\xi)=0$.
21 replies
Peter
Nov 1, 2005
Alphaamss
Mar 25, 2025
Derivative of function R^2 to R^2
Sifan.C.Maths   1
N Mar 22, 2025 by alexheinis
Source: Internet
Give a function $f:\mathbb{R}^2 \to \mathbb{R}^2: f(x,y)=(x^2+xy,y^2+x)$. Calculate the first and second derivative of the function at the point $(1,-1)$.
1 reply
Sifan.C.Maths
Mar 22, 2025
alexheinis
Mar 22, 2025
Derivative of Normalization Map has null space of dimension 1
myth17   4
N Mar 21, 2025 by myth17
Let $f(\vec{x}) = \frac{\vec{x}}{||\vec{x}||}$ be defined on $\mathbb{R}^n \setminus \{\vec{0}\}$. Show that the dimension of the kernel of $Df_{\vec{x}}$ for any $\vec{x} \in \mathbb{R}^n \setminus \{\vec{0}\}$ is $1$.
4 replies
myth17
Mar 20, 2025
myth17
Mar 21, 2025
Calculate the second derivative
harapan57   1
N Mar 19, 2025 by Mathzeus1024
Find $d^2z$ if $z=f(x,y)$ and $xy + z^2 -zx +zy - 2 = 0$.
1 reply
harapan57
Dec 3, 2021
Mathzeus1024
Mar 19, 2025
Cycle length of (x^2-1)/2x derivative relation power of two crazy
ehz2701   2
N Mar 1, 2025 by rchokler
Let
[center] $f(x) = \frac{x^2 - 1}{2x}$ [/center]

(this is the approximation using Newton’s method for $x^2 + 1 = 0$. Consider the sequence given by $a_{n+1} = f(a_n)$. Suppose for some $a_0$, we have the relationship $a_0 = a_\ell$, where $a_0 \neq a_i$ for $0<i<\ell$ (in other words, a cycle of length $\ell$). Show that


[center] $\frac{d}{dx}\, f^{(\ell)}(a_0) = 2^{\ell}$, [/center]

where $f^{(\ell)}(x)$ denotes $\ell$ function compositions of $f(x)$ (i.e. $f^{(\ell + 1)}(x) = f(f^{\ell}(x))$, $f^{(1)}(x) = f(x))$.)

—————

For example, when $x = \frac{1}{\sqrt{3}}$, it produces a cycle of length $2$, and $\left. \frac{d}{dx} f(f(x))\, \right|_{x=1/\sqrt{3}} = 2^2 = 4$
2 replies
ehz2701
Feb 28, 2025
rchokler
Mar 1, 2025
Differentiable function
kido2006   1
N Feb 18, 2025 by alexheinis
Let $ f:\left [ 0,+\infty  \right ] \rightarrow  \mathbb{R}$ be a twice differentiable function satisfying:
$$\lim_{x \to +\infty} (f''(x) - f'(x)) = b \neq 0.$$Prove that there exists $ x_0 $ such that $f(x) \neq 0, \quad \forall x \in (x_0, +\infty).$
1 reply
kido2006
Feb 18, 2025
alexheinis
Feb 18, 2025
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