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tamnd's digital brain — notes, problems, research

41641 notes

IMO 2021 Problem 1

The structure suggests a graph-theoretic or extremal-combinatorial reformulation.

imomathematicsolympiad
IMO 2020 Problem 6

The statement asks for a line that both separates a finite set of points and stays at distance at least on the order of $n^{-1/3}$ from every point.

imomathematicsolympiad
CF 1063308 - Атаки феникса

The problem describes Phoenix having an even number of coins. The coins have weights that are powers of two, starting from $2^1$ up to $2^n$.

codeforcescompetitive-programming
CF 1063306 - Камень, ножницы, бумага

The game has three participants: Fyodor, Matroskin, and Sharik. Each of them chooses one of the three classic moves: rock, paper, or scissors. A player wins only if their move defeats the moves of both other players at the same time.

codeforcescompetitive-programming
CF 1063307 - Ботать!

Sure. Please provide the Codeforces problem statement (or link, title, constraints, and any examples). I’ll write a detailed competitive programming editorial with: - Problem restatement - Key observations - Derivation of the algorithm - Proof of correctness - Complexity…

codeforcescompetitive-programming
CF 1063291 - Задача

Codeforces 1063291: Задача

codeforcescompetitive-programming
IMO 2020 Problem 5

Let the numbers on the cards be positive integers $a_1,\dots,a_n$.

imomathematicsolympiad
IMO 2020 Problem 4

The structure is purely combinatorial but constrained by monotonicity in altitude.

imomathematicsolympiad
IMO 2020 Problem 3

Working

imomathematicsolympiad
IMO 2020 Problem 2

The expression splits into two components: a linear form in $a,b,c,d$ and a multiplicative entropy-type term $a^a b^b c^c d^d$.

imomathematicsolympiad
IMO 2020 Problem 1

The problem concerns a convex quadrilateral $ABCD$ with an interior point $P$ whose position is constrained by two triples of angle ratios.

imomathematicsolympiad
CF 1063294 - Атака фениксов

I can write the full editorial, but the problem statement is missing from the prompt. The title “Codeforces 1063294 - Атака фениксов” is not enough to uniquely identify the task, and I do not want to invent the algorithm, examples, or code for the wrong problem.

codeforcescompetitive-programming
CF 1063292 - Баланс

I can write the full editorial in that format, but I’m missing the actual problem details. The identifier “Codeforces 1063292 - Баланс” does not map to a publicly available Codeforces problem I can reliably identify, and the “Problem Statement / Input / Output”…

codeforcescompetitive-programming
CF 1063293 - Незадача

I can write the full editorial in that format, but I need the actual problem statement for Codeforces 1063293 “Незада”.

codeforcescompetitive-programming
IMO 2019 Problem 6

The configuration is centered on the incircle of $ABC$ and its tangency triangle $DEF$.

imomathematicsolympiad
CF 106310E - Счастье в одной цифре

We are given a ticket represented by a string of digits whose length is always even. If we split this string into two equal halves, the ticket is considered “balanced” when the sum of digits in the left half equals the sum of digits in the right half.

codeforcescompetitive-programming
CF 106310D - Как развлекаются домашние роботы?

The game is played by three robots in a fixed order: Vacuum Cleaner, Kettle, and Speaker. They call four digit numbers one after another, but a player may have to try several numbers before making a valid move.

codeforcescompetitive-programming
CF 106310C - Магия слов

The task gives three lowercase English words. Eliza considers a phrase magical if the three words are all different and the phrase contains one or two of the special words see, believe, and repeat, but not all three of them at the same time.

codeforcescompetitive-programming
CF 106310B - Наклейки для скорой

I can’t reliably reconstruct a correct editorial for “Codeforces 106310B - Наклейки для скорой” from the information available here.

codeforcescompetitive-programming
IMO 2019 Problem 5

The system evolves a binary string of length $n$ under a deterministic rule depending on the current number of $H$’s.

imomathematicsolympiad
CF 106309E - Родительские хромосомы

We have a collection of parent chromosomes, each represented by a string of lowercase letters. When two parents create a child chromosome, every position of the child is copied independently from one of the two parents.

codeforcescompetitive-programming
CF 106309C - Грибочки

We are given a limited amount of dough and several types of buns we can bake. Each bun type consumes a fixed amount of dough, and for flavored buns it also consumes a limited amount of a specific filling. Each bun yields a fixed profit.

codeforcescompetitive-programming
CF 106309D - Тройки на доске

I can write the full editorial, but I need the actual problem statement details first. The prompt only includes the Codeforces title/ID and empty sections for statement/input/output, and I do not have enough information to reconstruct the problem reliably.

codeforcescompetitive-programming
CF 106309B - Карточная игра

using the style and formatting requirements you provided.

codeforcescompetitive-programming
CF 106309A - Могучие Промежуточники

I can write the editorial, but the problem statement fields in your prompt are empty and this problem has a nontrivial construction/search idea.

codeforcescompetitive-programming
IMO 2019 Problem 4

The expression on the right-hand side is

imomathematicsolympiad
CF 106296K - Hidden Digits

The problem gives a hidden sequence of decimal digits. In the non-interactive form, the whole digit string is available as input. We need determine whether there exists at least one contiguous part of this string whose numeric value is divisible by 3.

codeforcescompetitive-programming
CF 106296L - Robots

I can write the full editorial in that format, but the problem statement for Codeforces 106296L - Robots is missing from your prompt, and I could not identify the exact problem from the available information.

codeforcescompetitive-programming
CF 106296J - AND Components

We are given a collection of integers, and we are asked to understand connectivity induced by a bitwise rule. Each number can be thought of as a node in a graph.

codeforcescompetitive-programming
CF 106296I - Bobs Rating

I can write the editorial, but I need the exact accepted solution idea or a complete problem source for Codeforces 106296I first.

codeforcescompetitive-programming
CF 106296H - Magical Puzzles

We have a weighted tree. Each edge represents a piece of the path and has a cost. A journey between two vertices follows the unique path between them, and its distance is the sum of edge costs on that path. Some puzzles require collecting information from a set of edges.

codeforcescompetitive-programming
IMO 2019 Problem 3

Each operation replaces a “two-leaf star” centered at a vertex $A$, where $A$ is adjacent to $B$ and $C$ but $B$ and $C$ are not adjacent, by removing edges $AB$ and $AC$ and adding the edge $BC$.

imomathematicsolympiad
CF 106296G - Amoeba Tree

The structure we are dealing with is a rooted tree where every node is a branching point of a growing process. Each node represents an “amoeba state”, and edges describe parent to child transitions, meaning how a state can evolve into more refined states.

codeforcescompetitive-programming
CF 106296F - Binary Permutation

I can write the editorial, but I do not have enough verified information to produce a trustworthy solution for this problem yet.

codeforcescompetitive-programming
CF 106296D - Electi Lamps

I cannot provide a complete editorial for this problem in the requested format without inventing parts of the solution.

codeforcescompetitive-programming
CF 106296E - XOR Again?

We are given a static array of integers, and we conceptually split it into contiguous segments. For a fixed number of segments M, every valid partition of the array into exactly M consecutive blocks defines a cost.

codeforcescompetitive-programming
IMO 2019 Problem 2

The configuration contains a triangle $ABC$ with two free points $A_1 \in BC$ and $B_1 \in AC$, and two points $P \in AA_1$, $Q \in BB_1$ constrained by $PQ \parallel AB$.

imomathematicsolympiad
CF 106296B - Word Generator

I’m sorry, but I can’t provide a complete solution editorial and implementation for this problem in the requested format within the available response length.

codeforcescompetitive-programming
CF 106296C - Glitch

You’re right to flag this immediately. The identifier “106296C - Glitch” does not correspond to a standard Codeforces problem in the usual problemset index, and there is no accessible statement included in the prompt.

codeforcescompetitive-programming
CF 106296A - Magician

I’m sorry, but I can’t provide a full editorial with a complete solution for this specific competitive programming problem.

codeforcescompetitive-programming
IMO 2019 Problem 1

The equation couples values of $f$ at linear arguments with values of $f$ applied twice:

imomathematicsolympiad
CF 106284D - НОД-свёртка

We need count triples of non-negative integers (A, B, C) inside three given intervals such that the greatest common divisor of the two sums around B equals the sum of the two outer values: gcd(A + B, B + C) = A + C.

codeforcescompetitive-programming
CF 106284E - Игра Кроша

The game contains a line of carrot monsters. Krosh can only directly attack the first monster that is still alive. To reach a later monster, every monster before it must already be defeated. A shot aimed at the current monster deals increasing damage to a consecutive block.

codeforcescompetitive-programming
CF 106284C - Турнир Смешариков

We are given a complete round-robin tournament between $N$ players, where every pair of players plays exactly one match. Since there are $frac{N(N-1)}{2}$ matches in total, we need to output an ordering of all pairs.

codeforcescompetitive-programming
IMO 2018 Problem 6

The condition

imomathematicsolympiad
CF 106284A - Симфония будильников

The problem describes three alarm clocks that all start ringing at the same moment, time 0. The first clock rings every a minutes, the second every b minutes, and the third every c minutes.

codeforcescompetitive-programming
CF 106284B - Кто больше?

We have several unsigned integer types. A type with a bits can store values from 0 to 2^a - 1. We need decide whether there exists some integer x that fits into a smaller type, but its square does not fit into a larger type.

codeforcescompetitive-programming
CF 106282D - Великое тождество

I can write the editorial, but I need one missing piece first: the actual accepted solution idea/code for Codeforces 106282D - "Великое тождество" is not included in your prompt, and the statement alone is not enough to uniquely determine the intended editorial…

codeforcescompetitive-programming
CF 106282E - Модница

I can write the full editorial in that format, but I do not have enough information about Codeforces 106282E “Модница” to produce a correct solution.

codeforcescompetitive-programming
CF 106282C - Глиняная табличка

The input is a grid made of and . characters. You can think of it as a carved clay tablet where some cells are painted (stars) and others are empty (dots). On this grid, we slide a fixed window of size 5 rows by 3 columns across every valid position.

codeforcescompetitive-programming
IMO 2018 Problem 5

Let

imomathematicsolympiad
CF 106282A - Совунья и соления

The problem describes a simple situation: there are n mushrooms that need to be placed equally into k jars. If n is not divisible by k, some more mushrooms must be collected.

codeforcescompetitive-programming
CF 106282B - Числовые шестерёнки

Sure - send me the Codeforces problem statement (including constraints, examples, and preferably the intended difficulty if you know it), and I’ll write a detailed competitive programming editorial.

codeforcescompetitive-programming
CF 106281D - Глиняная табличка

The tablet is a rectangular grid of cells. A means the cell was painted, while a . means it was left empty. Every possible 5 x 3 window inside this grid may represent one digit.

codeforcescompetitive-programming
CF 106281E - Модница

I can write the editorial, but I’m missing the actual problem statement for Codeforces 106281E “Модница”.

codeforcescompetitive-programming
IMO 2018 Problem 4

The previous lower bound failed because it counted only Ben’s blue stones inside Amy’s chosen set and ignored Amy’s own red stones.

imomathematicsolympiad
CF 106281B - Числовые шестерёнки

I can’t see the actual statement of Codeforces 106281B (“Числовые шестерёнки”) in the material you provided, and it also doesn’t appear in the standard problemset index I can reliably reconstruct from memory alone.

codeforcescompetitive-programming
CF 106281C - Сонный Крош

The task is about producing the most profitable collection of buns from a limited amount of dough and several limited stuffing supplies. A bun can either have one of the available stuffings or have no stuffing at all.

codeforcescompetitive-programming
CF 106281A - Совунья и соления

I can write the editorial, but the problem statement you supplied is incomplete. I found that Codeforces Gym 106282 A "Совунья и соления" is the mushroom-and-jars problem: given n mushrooms and k jars, find the minimum additional mushrooms needed so the total is…

codeforcescompetitive-programming
CF 1062765 - Интересная эстафета

We have two teams of skiers moving toward each other on a straight track. Team A starts from point A, team B starts from point B, and the distance between the two points is d. Every skier moves with speed 1. The flag starts with the first skier from team A.

codeforcescompetitive-programming
CF 1062764 - Погрузка багажа

We are given a train of baggage carts connected one after another, numbered from the front (cart 1, attached to the tractor) to the back (cart n).

codeforcescompetitive-programming
CF 1062762 - Груша

The problem describes a balance scale with two pans and exactly three known weights. The weights have masses a, b, and c, with a < b < c. A pear of mass p has already been successfully measured, so we know that some arrangement of the three weights can balance it.

codeforcescompetitive-programming
CF 1062761 - Кофейня

I can write this editorial, but I need the exact problem statement/input-output details for Codeforces 1062761 - Кофейня.

codeforcescompetitive-programming
CF 1062763 - Два из трёх

I don’t have the actual statement of Codeforces 1062763 “Два из трёх” available from what you provided here, and the editorial depends heavily on the exact operations and constraints.

codeforcescompetitive-programming
CF 1062755 - Тайна священной последовательности

I can write the full editorial in that format, but the problem statement is missing. I could not reliably identify the exact task from only “Codeforces 1062755 - Тайна священной последовательности”, and inventing the algorithm would risk…

codeforcescompetitive-programming
CF 1062754 - Старая клавиатура

I can write the full editorial in that format, but I need the actual problem statement first. The title “Старая клавиатура” and the identifier “1062754” are not enough to uniquely identify a Codeforces problem from the available information, and the…

codeforcescompetitive-programming
CF 1062752 - Поездка на олимпиаду

I don’t yet have enough reliable information about the actual statement of Codeforces 1062752 - “Поездка на олимпиаду” to produce a correct, problem-specific editorial.

codeforcescompetitive-programming
CF 1062751 - Плацкартный вагон

Sure - send me the Codeforces problem statement (or link/text), and I’ll write a detailed competitive programming editorial.

codeforcescompetitive-programming
CF 1062753 - Стритбольная команда

I can write the full editorial in that format, but I do not have the actual problem statement for Codeforces 1062753 “Стритбольная команда”.

codeforcescompetitive-programming
CF 1062745 - Древний свиток

The scroll contains a sequence of decimal digits. A fragment of the scroll is chosen by giving its left and right positions. For that fragment, we look at every ordered pair of two different positions.

codeforcescompetitive-programming
CF 1062744 - Врата кибернетиков

We have a string made only of the characters 1 and 2. The amount of 1 characters is exactly the same as the amount of 2 characters. The gate is stable only when equal characters never stand next to each other, meaning the final string must alternate like 121212... or 212121....

codeforcescompetitive-programming
CF 1062742 - Инопланетные часы

We are working with a non-standard clock system defined by two parameters: the number of hours in a day h and the number of minutes in an hour m. A moment of time is given as a pair (x, y) where x is the current hour and y is the current minute, with 0 ≤ x < h and 0 ≤ y < m.

codeforcescompetitive-programming
CF 1062741 - Освещение комнаты

The problem compares two light bulbs. The ordinary bulb is cheaper to buy but consumes more electricity. The energy saving bulb costs more at the beginning, but every hour it uses fewer watts.

codeforcescompetitive-programming
CF 1062743 - Чернильная история

I can write the editorial, but the problem statement you included is empty and I needed the actual statement to avoid inventing details.

codeforcescompetitive-programming
CF 1062734 - Раскраска стены

We have a rectangular wall divided into tiles. Some positions contain lamps and cannot be painted. Every other position must receive one of k colors.

codeforcescompetitive-programming
CF 1062735 - Проблемы логистики

I can write the editorial, but I need the actual problem statement or enough details about Codeforces 1062735 “Проблемы логистики” first.

codeforcescompetitive-programming
CF 1062731 - Рекламные паузы

We are given a timeline of temperatures indexed by integer days, where day 0 is today. The input provides a continuous sequence of observed temperatures for the past N days, today, and the next N days.

codeforcescompetitive-programming
CF 1062733 - Встреча у фонтана

I can write the editorial in the requested format, but I’m missing the actual problem statement for Codeforces 1062733 “Встреча у фонтана”.

codeforcescompetitive-programming
CF 1062732 - Популярный пост

I can write the full editorial in that format, but I need the actual problem statement first. The identifier “Codeforces 1062732 - Популярный пост” does not provide enough information to reconstruct the task reliably, and I do not want to invent the algorithm…

codeforcescompetitive-programming
CF 1062712 - План эвакуации

Codeforces 1062712: План эвакуации

codeforcescompetitive-programming
CF 1062711 - Большой квадрат

Codeforces 1062711: Большой квадрат

codeforcescompetitive-programming
CF 1062715 - Шифр mint

I cannot reliably reconstruct the full original Codeforces statement for “1062715 - Шифр mint” from public archives or standard problem indexes, and there is no accessible statement text in the prompt.

codeforcescompetitive-programming
CF 1062714 - Гравитационная сортировка

We are given a system that transforms two arrays into a 2D pattern using a very specific construction. Each index i produces a horizontal segment: starting from column 1 up to column p[i], we place blocks of color c[i] across that entire segment in row i.

codeforcescompetitive-programming
CF 1062713 - Аркадий Аркадьевич делает грядки

I can’t responsibly write a correct editorial yet because the actual problem statement for Codeforces 1062713 - “Аркадий Аркадьевич делает грядки” is not available in your prompt, and it is not a standard Codeforces problem with a reliably…

codeforcescompetitive-programming
CF 106269A - Как вы яхту назовёте, так она и поплывёт

Sure. Please provide the Codeforces problem statement (or a link/name), and I’ll write a detailed competitive programming editorial covering: - Problem restatement and intuition - Key observations - Derivation of the algorithm - Proof of correctness - Complexity analysis -…

codeforcescompetitive-programming
CF 106269E - Мы в некотором роде не совсем гавайцы

with a full correctness argument, implementation details, example traces, and tested Python code.

codeforcescompetitive-programming
CF 106269D - Руки вверх! Ваша песенка спета!

I can’t write a correct editorial for this yet because the actual problem statement is missing from your prompt. “Codeforces 106269D - Руки вверх! Ваша песенка спета!

codeforcescompetitive-programming
CF 106269B - Похищение века

I can't write a correct editorial for this problem because the actual problem statement, input format, and output format are missing from your prompt. For Codeforces Gym problem 106269B, the title alone is not enough to reconstruct the task.

codeforcescompetitive-programming
CF 106269C - Полный назад! Самый полный назад!

I can’t write a correct editorial for this yet because the problem statement is missing. Right now I only see the title “Codeforces 106269C - Полный назад! Самый полный назад!

codeforcescompetitive-programming
CF 106260J - 排列

We are building strings of length $n$ using exactly four characters: $y, u, k, o$. The restriction is local: if you ever look at any consecutive block of 5 characters, that block must not be a permutation of the multiset formed by the letters of the word “yuuko”.

codeforcescompetitive-programming
CF 106260K - 调和级数求和

I could not reliably recover the actual statement of Codeforces Gym 106260K from available sources. The title "调和级数求和" only indicates a harmonic-series summation problem, but the input format, required output, constraints, and exact quantity to compute are missing.

codeforcescompetitive-programming
CF 106260H - 猫树

following your exact style and formatting requirements.

codeforcescompetitive-programming
CF 106260G - seven

I can't reliably write a correct editorial for Codeforces 106260G - seven because the problem statement is missing from your prompt, and that Gym problem is not identifiable from the information provided. Please send one of the following: 1. The full problem statement, or 2.

codeforcescompetitive-programming
CF 106260E - five

I could not reliably retrieve the exact statement of Codeforces 106260E - five from public sources, which usually means it is either an archived problem with limited indexing or a contest-specific statement not fully mirrored outside Codeforces.

codeforcescompetitive-programming
CF 106260F - six

following all the style and formatting constraints you provided.

codeforcescompetitive-programming
IMO 2017 Problem 6

The earlier normalization attempt failed because the separated values were arbitrary nonzero integers and not necessarily invertible.

imomathematicsolympiad
CF 106260C - backpack

We are given a directed graph of caverns and tunnels. We start in cavern 1 with zero rocks in the backpack and want to reach cavern n. A tunnel adds a rocks when we enter it, then removes b rocks when we leave it.

codeforcescompetitive-programming
CF 106260D - four

I can write the editorial, but I need the actual problem details first. The statement section in your prompt is empty, and I could not reliably identify the problem from only “Codeforces 106260D - four”.

codeforcescompetitive-programming
CF 106260B - 回文

We are given a rooted tree where each node carries a lowercase letter. The root is node 1, and every node has a well-defined depth from the root. Each query focuses on one node u and a target depth d.

codeforcescompetitive-programming
CF 106260A - 树,排列,期望

Sure - send me the Codeforces problem statement (or link, if you have it), and I’ll write a detailed editorial in a competitive programming style.

codeforcescompetitive-programming