Number Theory

by AnhQuang_67, Apr 24, 2025, 1:44 PM

Let $p$ be an even prime number and a sequence $\{a_n\}_{n=1}^{+\infty}$ satisfy $$a_1=1, a_2=2$$and $$a_{n+2}=2\cdot a_{n+1}+3\cdot a_n, \forall n \geqslant 1$$Prove that always exists positive integer $k$ satisfying for all positive integers $n$, then $a_n \ne k \mod{p}$.

P/s: $\ne$ is "not congruence"

Irrational equation

by giangtruong13, Apr 24, 2025, 1:44 PM

Solve the equation : $$(\sqrt{x}+1)[2-(x-6)\sqrt{x-3}]=x+8$$
This post has been edited 1 time. Last edited by giangtruong13, 30 minutes ago

Classic graph theory lemma?

by eulerleonhardfan, Apr 24, 2025, 1:29 PM

$n \in \mathbb{N}$ is given, $A$, $B$ are graphs on the same set of $n$ nodes, having $a, b$ connected components respectively. Prove that $A \cup B$ has at least $a+b-n$ connected components.

Inspired by 2024 Fall LMT Guts

by sqing, Apr 24, 2025, 12:24 PM

Let $x$, $y$, $z$ are pairwise distinct real numbers satisfying $x^2+y =y^2 +z = z^2+x. $ Prove that
$$(x+y)(y+z)(z+x)=-1$$Let $x$, $y$, $z$ are pairwise distinct real numbers satisfying $x^2+2y =y^2 +2z = z^2+2x. $ Prove that
$$(x+y)(y+z)(z+x)=-8$$
This post has been edited 2 times. Last edited by sqing, 2 hours ago

Maximum number of nice subsets

by FireBreathers, Apr 23, 2025, 10:27 PM

Given a set $M$ of natural numbers with $n$ elements with $n$ odd number. A nonempty subset $S$ of $M$ is called $nice$ if the product of the elements of $S$ divisible by the sum of the elements of $M$, but not by its square. It is known that the set $M$ itself is good. Determine the maximum number of $nice$ subsets (including $M$ itself).

Dividing Pairs

by Jackson0423, Apr 13, 2025, 8:39 AM

Let \( a \) and \( b \) be positive integers.
Suppose that \( a \) is a divisor of \( b^2 + 1 \) and \( b \) is a divisor of \( a^2 + 1 \).
Find all such pairs \( (a, b) \).

circle geometry showing perpendicularity

by Kyj9981, Mar 18, 2025, 11:53 AM

Two circles $\omega_1$ and $\omega_2$ intersect at points $A$ and $B$. A line through $B$ intersects $\omega_1$ and $\omega_2$ at points $C$ and $D$, respectively. Line $AD$ intersects $\omega_1$ at point $E \neq A$, and line $AC$ intersects $\omega_2$ at point $F \neq A$. If $O$ is the circumcenter of $\triangle AEF$, prove that $OB \perp CD$.

Min Number of Subsets of Strictly Increasing

by taptya17, Dec 13, 2024, 8:24 AM

Let $n$ be a positive integer. Initially the sequence $0,0,\cdots,0$ ($n$ times) is written on the board. In each round, Ananya choses an integer $t$ and a subset of the numbers written on the board and adds $t$ to all of them. What is the minimum number of rounds in which Ananya can make the sequence on the board strictly increasing?

Proposed by Shantanu Nene

Ellipse and Vectors

by scls140511, Sep 8, 2024, 1:15 PM

7 Let $F_1$ and $F_2$ be the two foci of ellipse $\omega$. $P$ is a point on $\omega$. Let $O$ be the center of the excircle of $\triangle PF_1F_2$. When $\vec{PO} \cdot \vec{F_1F_2} = 2\vec{PF_1} \cdot \vec{PF_2}$, find the minimum eccentricity of $\omega$.
This post has been edited 1 time. Last edited by scls140511, Sep 8, 2024, 1:15 PM

Nice inequality

by sqing, Apr 24, 2019, 1:01 PM

Let $a_1,a_2,\cdots,a_n  (n\ge 2)$ be real numbers . Prove that : There exist positive integer $k\in \{1,2,\cdots,n\}$ such that $$\sum_{i=1}^{n}\{kx_i\}(1-\{kx_i\})<\frac{n-1}{6}.$$Where $\{x\}=x-\left \lfloor x \right \rfloor.$

A blog documenting a (no longer) high school youth and his struggles with advancing his mathematical skill.

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  • dj so orz :omighty:

    by Yiyj1, Mar 29, 2025, 1:42 AM

  • legendary problem writer

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  • orz $$\,$$

    by balllightning37, Jul 26, 2024, 1:05 AM

  • hi dj $ $ $ $

    by OronSH, Jul 23, 2024, 2:14 AM

  • i wanna submit my own problems lol

    by ethanzhang1001, Jul 20, 2024, 9:54 PM

  • hi dj, may i have the role of contributer? :D

    by lpieleanu, Feb 23, 2024, 1:31 AM

  • This was helpful!

    by YIYI-JP, Nov 23, 2023, 12:42 PM

  • waiting for a recap of your amc proposals for this year :D

    by ihatemath123, Feb 17, 2023, 3:18 PM

  • also happy late bday man! i missed it by 2 days but hope you are enjoyed it

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  • Contrib? :D

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  • :love: tfw kakuro appears on amc :love:

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  • Hi dj :)

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  • Roses are red,
    Wolfram is banned,
    The best problem writer is
    Djmathman

    by ihatemath123, Aug 6, 2022, 12:19 AM

  • hello :)

    by aidan0626, Jul 26, 2022, 5:49 PM

  • Do you have a link to your main blog that you started after graduating from high school, I couldn't find it. @dj I met you IRL at Awesome Math summer Program several years ago.

    by First, Mar 1, 2022, 5:18 PM

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