• Steinhaus theorem

    Submitted by yonatan zilpa on

    Let $\lambda$ be Borel measure.
    1. If $A$ is mesuarable and $\lambda(A)>0$, then $A-A=\left\{ x-y\; :\; x,y\in A \right\}$ contains a segment $I$ such that $0\in I$
    2. If $A, B$ are measurable sets and $\lambda(A), \lambda(B)>0$, then $A+B$ contains a segment I.
    Tags: 
    theorems

  • The circle of Apollonius

    Submitted by yonatan zilpa on

    By definition, a circle is the locus of all points equidistant from a fixed center. However, the circle of Apollonius is defined differently. In the following post I will define and construct an Apollonian circle.
    Tags: 
    geometry

  • Hahn Decomposition Theorem

    Submitted by yonatan zilpa on

    Let $\nu$ be a signed measure over measurable space $(X,\mathcal{M})$. Denote $\tilde{N}=\left\{ u\in \mathcal{M} \; :\; \mbox{$u$ is $\nu$-negative} \right\}$ and $\tilde{P}=\left\{ u\in \mathcal{M} \; :\; \mbox{$u$ is $\nu$-positive} \right\}$
    1.   $\exists N\in \tilde{N}\Big(\exists P \in \tilde{P}\Big( N\cap P=\emptyset \; \mbox{ and }\; N\cup P = X\Big)\Big)$
    2.   $\forall N,N^{'}\in \tilde{N}\Big(\forall P,P^{'}\in \tilde{P} \Big(N\sqcup P =N^{'}\sqcup P^{'}=X \; \Longrightarrow \nu( P\triangle \tilde{P})=\nu( N\triangle \tilde{N})=0\Big)\Big) $
    Tags: 
    theorems

  • Minimum way to move all circles

    Submitted by yonatan zilpa on

    Moving circles
    Consider a game with three vertical sticks and 5 circles, where each circle has a hole in the middle. The circles arranged in one stick as shown in the picture: The task is to move all the circles to a nearby stick, circles can be move from one stick to another, but a circle cannot be put over a smaller one. What are the minimum number of steps needed to complete this task?
Tags: 
recreational mathematics

  • Asymptotic Differentiable Function

    Submitted by yonatan zilpa on

    Let $f$ be any function asymptotic at zero. Prove/Disprove:
    If $f$ is differentiable everywhere in its domain, then $\lim_{x\to \infty}f\;'(x)$ must be equal to zero?
    Tags: 
    recreational mathematics

  • Conceptual Language and Learning

    Submitted by yonatan zilpa on

    Normally when we learn human spoken language we translate words to our own mother tongue language. But when we learn our mother tongue language we have no language to translate new words. How do we understand mother tongue without translating words? Is there some kind of conceptual language where we can translate mother tongue new words? And if so, can we use this conceptual language to become better learner?

    concept map

    Tags: 
    learning tips

  • Four bottles - Equidistant mouth problems

    Submitted by yonatan zilpa on

    How to place four identical bottles in the plane such that the distance between each pair of mouths will be the same?
    Tags: 
    math recriation

  • Learning tips

    Submitted by yonatan zilpa on

    How to make the most of your learning time? Efficient learning and good usage of time may be key ingredients. Each student may have his own unique style of learning, but there exist patterns and strategies that can help everyone. Here is a list of links to good webinars and lectures note that may help you chose your own style of learning.
    Tags: 
    learning tips