• ## Green Boy Can't You Solve It Out? Ep. 2

Solved! Difficulty: Div2B 1 problem for div3 participants. Beautiful Dice Painting When Epis is bored, she will draw dices. The canvas contains n*n dots. Initially they are all white. You need to paint exactly v dots black so that the final c……
XGN 2023年10月22日
• ## Crimboi [14] 开门红(?)

题目背景 Rudin Real and Complex Analysis第一题, 真是开门红. 题面 We call a pairing (X,\mathfrak{M}) \sigma-algebra if \mathfrak{M} is a subset of P(X) satisfying the three properties below: (1) X\in\mathfrak{M} (2) If A\in\mathfrak{M}, A^c\in\mathf……
HDD 2023年10月11日
• ## Orange Boy Can You Solve It Out? Ep. 57

trivial 思考题 I am Banach This is a classic problem by Banach nearly without any change. There are N boys and M girls. Each boy loves a single girl and each girl loves a single boy. Your task is to divide the boys into two groups A_0 and A_1 and t……
XGN 2023年10月7日
• ## Announcement on Solutions for Orange Boy Can You Solve It Out

Starting from today, we will try to write tutorials and rate difficulties for already solved Orange Boy Can You Solve It Out?(OBCYSIO) problems and separate them from unsolved/unsolvable ones. There's a new category called Solved OBCYSIO and you ca……
XGN 2023年10月6日
• ## Orange Boy Can You Solve It Out? Ep. 56

思考题 during OI recovery Rubbish Problem Maybe you have heard of the evil problem Hilbert's Hedge Maze and here's another very fun problem! You have n items arranged in rows. Each row contains r items and the items are arranged from left to right,……
XGN 2023年10月6日
• ## Crimson boi can u water it [13] 几把水题

题目背景 南大几把选拔中最水的题(T3), 适合出在初中竞赛里. 另外这期写解答方便以后想考南大几把的学生?, 篇幅太长除了题面不写中文了. 不过题面也是pmt提供的, 不一定完全准确. 题面 记排列P的置换环数量为c(P),排列P,Q的复合记为P\oplus Q，已知长度为n的排列f_1,f……
HDD 2023年9月6日
• ## Orange Boy Can You Solve It Out? Ep. 55

思考题 with hard-to-write checker and interactor Peripheral Lightup! Note: this is not a traditional problem. Follow the instruction in the statement to help construct your solution Lead-in: The Palace is celebrating a festival with LED lights! Ins……
XGN 2023年7月20日
• ## 英語読解問題（嘘）

XGN 2023年6月4日
• ## Orange Boy Can You Solve It Out? Ep. 54

Solved! Difficulty: Div2C 思考题 in dreamland Construct it XGN dreamt about the following problem when sleeping: You are given a positive integer n (2\leq n\leq 10^8). Output an arithmetic expression that yields the value n with the following……
XGN 2023年6月3日
• ## Crimson-boi[12] 脑筋急转弯

题目背景 贴吧范畴论帅哥问的一道题, 题目还算基础, 不过确实很容易伪证, 是名副其实的脑筋急转弯. 题面 对一个拓扑空间X, 定义集合S为稠密集当且仅当\text{Cl}(S)=X, R为无处稠密集当且仅当X-\text{Cl}(R)为稠密集. 定义集合A的边界\text{Bd}(A)=\text{Cl}(A)\cap\te……
HDD 2023年5月27日