回顾这个 blog 的历史,一开始是基于 Jekyll,后来因为太慢了,换成了 Hexo。今年我又折腾了一下 Hexo,例如加了中文字体切割,把 Related Posts 换成了基于 embedding model 的 recommendation,但是换来换去感觉还是基于别人的 theme 做。我还粗略看了一下其他的 SSG,例如 Hugo 的 PaperMod,但是感觉还是不如 hexo-theme-next

后来也趁着 vibe coding 这么流行,我也在想要不要借此机会试试 vibe coding。之前的 feature 也都是 vibe code 出来的,我懒得去看 Hexo 的文档了(而且有些地方文档还不清楚)。这次就趁着大改 theme 的机会多试试 vibe coding(主要使用 Claude Code)。

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今年的 SIGYAO30 就决定放在 Denver 啦,

2025 年的 第一场 SIGYAO30,比以往时候鸽的更多一些,最后只有十一个人过来了……东岸的人尤为鸽,为什么鸽也能出现了人传人现象?汤姐一如既往的鸽,她吐槽为什么每次 SIGYAO30 都能精准选在她生理期。胡老师本来非常积极,但是出发前一天,胡老师吃坏肚子了……令人惊奇的是鸽王 xpd 居然来了,另外一个亮点是毕老师不远千里迢迢从 HK 赶过来了,第一次参加线下的 SIGYAO30。

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几个星期前,一位小伙伴 wzc 问我有啥娱乐活动。聊着聊着,我们就约起了 Yosemite。于是几天前,我们终于趁着还不需要预约的时候,赶个 ddl 去了趟 Yosemite.

TL;DR 实在是妙啊!不愧是国家公园啊!瀑布量大管饱。

先放一张 Yosemite 的地图,我觉得这张地图是最有用的:原图在这里

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题面

一个集合 SS 被称为 sum-free set 当且仅当 a,bS,a+b∉S\forall a, b \in S, a + b \not\in S

试证明:对于任何一个集合 AZ\{0}A \subseteq \ZZ\backslash\{0\},其最大的 sum-free subset 大小至少为 A/3|A| / 3.

(有一个和题面差不多长度的解答)

解答

对于 tU[0,1]t \sim U[0, 1],令 At:={xA:1/3<xtmod1<2/3}A_t := \{x \in A: 1/3 < xt \bmod 1 < 2/3 \}。显然 AtA_t 为 sum-free set. 最后再注意到 maxtAtEt[At]=A/3\max_t |A_t| \geq \mathbb{E}_t [|A_t|] = |A|/3 即可。

解法来自 Erdos。

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In this post, we are interested in finding all integer and rational solutions for a general binary quadratic equation:

ax2+bxy+cy2+dx+ey+f=0,\labeleq:general\begin{equation} ax^2 + bxy + cy^2 + dx + ey + f = 0, \label{eq:general} \end{equation}

for any integer coefficients a,b,c,d,e,fa, b, c, d, e, f. The equation might degenerate; we only point it out when it does but do not solve it, as it’s typically easier to solve a degenerate equation.

This post mainly serves as an entry point for a series of posts and also contains a little bit of the story behind the posts. That is why this post has index 0 in the title.

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In this post, we are interested in finding all integer solutions of the following equation:

ax2+bxy+cy2=n.\begin{equation} ax^2 + bxy + cy^2 = n. %\label{eq:ibqf} \end{equation}

where acn0acn \neq 0, b24ac0b^2 - 4ac \neq 0. This equation can certainly be solved using methods from the generalized Pell equation, but here, we explore other methods.

The function ax2+bxy+cy2ax^2 + bxy + cy^2 is also called the binary quadratic form (BQF), which we denote as <a,b,c>\left< a,b,c \right> for brevity. The discriminant of <a,b,c>\left< a,b,c \right> is defined as Δ:=b24ac\Delta := b^2 - 4ac.

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In this post, we are interested in finding all integer solutions of Pell equation

x2dy2=1,\labeleq:pell\begin{equation} x^2 - d y^2 = 1, \label{eq:pell} % \tag{Pell Equation} \end{equation}

and generalized Pell equation

x2dy2=n,\labeleq:generalizedpell\begin{equation} x^2 - d y^2 = n, \label{eq:generalized-pell} % \tag{Generalized Pell Equation} \end{equation}

for a non-square integer d>0d > 0 and n0n \neq 0.

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In this post, we’re interested in finding any or all rational solutions of the following equation:

ax2+by2+c=0,ax^2 + by^2 + c = 0,

or alternatively, all integer solutions to the Legendre equation:

ax2+by2+cz2=0,\labeleq:legendre\begin{equation} ax^2 + by^2 + cz^2 = 0, \label{eq:legendre} % \tag{Legendre Equation} \end{equation}

where abc0abc \neq 0.

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