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tamnd's digital brain — notes, problems, research

42616 notes

IMO 1992 LL KOR47

Find the largest integer not exceeding $1992

imolonglistmathematicsolympiad
IMO 1989 LL GRE29

Let L denote the set of all lattice points of the plane (points

imolonglistmathematicsolympiad
IMO 1987 LL GBR26

Prove that if x, y, z are real numbers such that x2+y2+z2 = 2,

imolonglistmathematicsolympiad
IMO 1988 LL USS86

Let a, b, c be integers different from zero. It is known that the

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IMO 1992 LL FIN16

Find all triples (x, y, z) of integers such that

imolonglistmathematicsolympiad
IMO 1982 LL BRA10

Let r1, . . . , rn be the radii of n spheres. Call S1, S2, . . . , Sn the

imolonglistmathematicsolympiad
IMO 1976 LL USS44

A circle of radius 1 rolls around a circle of radius

imolonglistmathematicsolympiad
IMO 1967 LL GBR19

The n points P1, P2, . . . , Pn are placed inside or on the bound-

imolonglistmathematicsolympiad
IMO 1985 LL TUR80

Let E = {1, 2, . . ., 16} and let M be the collection of all

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IMO 1992 LL PRK59

Let a regular 7-gon A0A1A2A3A4A5A6 be inscribed in a circle.

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IMO 1989 LL HKG35

Find all square numbers S1 and S2 such that S1 −S2 = 1989.

imolonglistmathematicsolympiad
IMO 1985 LL VIE97

In a plane a circle with radius R and center w and a line 
ambda

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IMO 1979 LL BEL2

For a finite set E of cardinality n \geq3, let f(n) denote the

imolonglistmathematicsolympiad
IMO 1972 LL BUL3

On a line a set of segments is given of total length less than

imolonglistmathematicsolympiad
IMO 1987 LL VIE72

Is it possible to cover a rectangle of dimensions m imes n with

imolonglistmathematicsolympiad
IMO 1967 LL HUN25

Three disks of diameter d are touching a sphere at their centers.

imolonglistmathematicsolympiad
IMO 1970 LL GDR29

Prove that the equation 4x+6x = 9x has no rational solutions.

imolonglistmathematicsolympiad
IMO 1988 LL USS87

All the irreducible positive rational numbers such that the prod-

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IMO 1985 LL SPA74

Find the triples of positive integers x, y, z satisfying

imolonglistmathematicsolympiad
IMO 1992 LL FRG19

Denote by an the greatest number that is not divisible by 3

imolonglistmathematicsolympiad
IMO 1992 LL JAP41

Let S be a set of positive integers n1, n2, . . . , n6 and let n(f)

imolonglistmathematicsolympiad
IMO 1988 LL INA41

(a) Let ABC be a triangle with AB = 12 and AC = 16. Suppose M is the

imolonglistmathematicsolympiad
IMO 1992 LL TUR74

Let S =

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IMO 1979 LL BRA8

The sequence (an) of real numbers is defined as follows:

imolonglistmathematicsolympiad
IMO 1983 LL POL49

Given positive integers k, m, n with km \leqn and nonnegative

imolonglistmathematicsolympiad
IMO 1992 LL VIE82

Let f(x) = xm + a1xm−1 + \cdot \cdot \cdot + am−1x + am and g(x) =

imolonglistmathematicsolympiad
IMO 1969 LL GBR26

A smooth solid consists of a right circular cylinder of height

imolonglistmathematicsolympiad
IMO 1978 LL NET31

Let the polynomials

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IMO 1982 LL FRA29

Let f : R oR be a continuous function. Suppose that the

imolonglistmathematicsolympiad
IMO 1978 LL VIE51

Find the relations among the angles of the triangle ABC whose

imolonglistmathematicsolympiad
IMO 1979 LL USA65

Given f(x) \leqx for all real x and

imolonglistmathematicsolympiad
IMO 1987 LL USS67

If a, b, c, d are real numbers such that a2 + b2 + c2 + d2 \leq1,

imolonglistmathematicsolympiad
IMO 1979 LL USA64

From point P on arc BC of the circumcircle about triangle

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IMO 1976 LL USS42

For a point O inside a triangle ABC, denote by A1, B1, C1

imolonglistmathematicsolympiad
IMO 1989 LL MON67

A family of sets A1, A2, . . . , An has the following properties:

imolonglistmathematicsolympiad
IMO 1984 LL GBR26

A cylindrical container has height 6 cm and radius 4 cm. It

imolonglistmathematicsolympiad
IMO 1989 LL SWE94

Prove that a < b implies that a3 −3a \leqb3 −3b + 4. When

imolonglistmathematicsolympiad
IMO 1992 LL USA80

Given a graph with n vertices and a positive integer m that is

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IMO 1972 LL USS43

A fixed point A inside a circle is given. Consider all chords

imolonglistmathematicsolympiad
IMO 1986 LL ISR49

Let C1, C2 be circles of radius 1/2 tangent to each other and

imolonglistmathematicsolympiad
IMO 1982 LL CZS21

All edges and all diagonals of regular hexagon A1A2A3A4A5A6

imolonglistmathematicsolympiad
IMO 1986 LL USS77

Find all integers x, y, z that satisfy

imolonglistmathematicsolympiad
IMO 1992 LL FIN15

Prove that there exist 78 lines in the plane such that they have

imolonglistmathematicsolympiad
IMO 1971 LL NET30

Prove that the system of equations

imolonglistmathematicsolympiad
IMO 1986 LL IRE45

Given n real numbers a1 \leqa2 \leq\cdot \cdot \cdot \leqan, define

imolonglistmathematicsolympiad
IMO 1970 LL BUL15

Given a triangle ABC, let R be the radius of its circumcir-

imolonglistmathematicsolympiad
IMO 1966 LL BUL21

Prove that the volume V and the lateral area S of a right circular

imolonglistmathematicsolympiad
IMO 1992 LL SPA66

A circle of radius ho is tangent to the sides AB and AC of the

imolonglistmathematicsolympiad
IMO 1985 LL CAN12

Find the maximum value of

imolonglistmathematicsolympiad
IMO 1966 LL CZS26

(a) Prove that (a1 +a2 +\cdot \cdot \cdot+ak)2 \leqk(a2

imolonglistmathematicsolympiad
IMO 1979 LL FRA24

Let a and b be coprime integers, greater than or equal to 1.

imolonglistmathematicsolympiad
IMO 1983 LL CAN18

Let b \geq2 be a positive integer.

imolonglistmathematicsolympiad
IMO 1970 LL ROM43

Prove that the equation

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IMO 1988 LL BUL2

Let an =

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IMO 1969 LL POL57

On the sides AB and AC of triangle ABC two points K and

imolonglistmathematicsolympiad
IMO 1985 LL VIE96

Determine all functions f : R oR satisfying the following two

imolonglistmathematicsolympiad
IMO 1985 LL MON52

In the triangle ABC, let B1 be on AC, E on AB, G on BC,

imolonglistmathematicsolympiad
IMO 1972 LL GBR16

Consider the set S of all the different odd positive integers

imolonglistmathematicsolympiad
IMO 1973 Problem 3

The polynomial

imomathematicsolympiad
IMO 1973 Problem 2

The reviewers correctly identified that the previous proof failed at the planar lemma.

imomathematicsolympiad
IMO 1973 Problem 1

The reviewer identified a false claim in the previous proof:

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IMO 1972 Problem 6

The problem asks for the existence of a regular tetrahedron with one vertex on each of four given distinct parallel planes in $\mathbb{R}^3$.

imomathematicsolympiad
IMO 1972 Problem 4

The system involves five positive real numbers $(x_1, x_2, x_3, x_4, x_5)$ linked cyclically by inequalities of the form $(x_i^2 - x_{i+2}x_{i+4})(x_{i+1}^2 - x_{i+2}x_{i+4}) \le 0$, where indices are…

imomathematicsolympiad
IMO 1972 Problem 3

Consider small values of $m$ and $n$ to examine the expression

imomathematicsolympiad
IMO 1972 Problem 2

A cyclic quadrilateral is given.

imomathematicsolympiad
IMO 1972 Problem 1

Consider a set of ten distinct two-digit numbers, $S = {a_1, a_2, \dots, a_{10}}$, and examine the sums of all its non-empty subsets.

imomathematicsolympiad
IMO 1971 Problem 6

The reviewer identified only one critical flaw, namely the final deduction from

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IMO 1971 Problem 5

For small values of $m$, explicit examples suggest a graph-theoretic interpretation.

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IMO 1971 Problem 4

The path $XYZTX$ lies on the four faces adjacent cyclically around the tetrahedron.

imomathematicsolympiad
IMO 1971 Problem 3

The sequence begins

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IMO 1971 Problem 2

Let $P_1$ be a convex polyhedron with nine vertices $A_1,\dots,A_9$ in $\mathbb{R}^3$, and let $P_i = P_1 + (A_i - A_1)$ for $i=2,\dots,9$.

imomathematicsolympiad
IMO 1971 Problem 1

Define

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IMO 1959 Problem 1

The fraction

imomathematicsolympiad
LeetCode 2547 - Minimum Cost to Split an Array

The problem asks us to split the array nums into one or more contiguous non-empty subarrays such that the total cost is minimized. For every subarray, we define a special quantity called its importance value.

leetcodehardarrayhash-tabledynamic-programmingcounting
LeetCode 2751 - Robot Collisions

This problem describes a simulation of robots moving on a one-dimensional line, each with a given starting position, health, and direction. Every robot moves at the same speed, and collisions occur when two robots meet at the same position.

leetcodehardarraystacksortingsimulation
LeetCode 2501 - Longest Square Streak in an Array

The problem asks us to identify the longest square streak in an array of integers nums. A square streak is a subsequence of at least length 2 where, after sorting, every element is the square of the previous element. In other words, if the sorted subsequence is [x1, x2, ...

leetcodemediumarrayhash-tablebinary-searchdynamic-programmingsorting
CF 245H - Queries for Number of Palindromes

We are given a fixed string consisting of lowercase letters, and many independent queries. Each query specifies a contiguous segment of this string, and for that segment we must count how many substrings are palindromes.

codeforcescompetitive-programmingdphashingstrings
LeetCode 2762 - Continuous Subarrays

That is a long, structured technical guide. To make sure I target the correct problem and avoid wasting your time, can you confirm the exact LeetCode problem title for 2762?

leetcodemediumarrayqueuesliding-windowheap-(priority-queue)ordered-setmonotonic-queue
LeetCode 1970 - Last Day Where You Can Still Cross

This problem asks us to find the last day we can cross a grid from the top row to the bottom row, walking only on land. The grid is initially all land (0), and each day, specific cells are flooded with water (1) according to the cells array.

leetcodehardarraybinary-searchdepth-first-searchbreadth-first-searchunion-findmatrix
LeetCode 3007 - Maximum Number That Sum of the Prices Is Less Than or Equal to K

The problem asks us to find the largest integer num such that the accumulated price of all numbers from 1 to num does not exceed a given threshold k.

leetcodemediummathbinary-searchdynamic-programmingbit-manipulation
LeetCode 1761 - Minimum Degree of a Connected Trio in a Graph

This problem gives us an undirected graph with n nodes and a list of edges. A connected trio is a group of exactly three distinct nodes where every pair of nodes has an edge between them. In graph theory terms, this is simply a triangle.

leetcodehardgraph-theoryenumeration
CF 245B - Internet Address

Vasya scribbled an Internet address in his notebook, but he was in a hurry and omitted all punctuation characters like :, /, and ..

codeforcescompetitive-programmingimplementationstrings
CF 431C - k-Tree

We are working with a rooted infinite tree where every node always has exactly $k$ outgoing edges to children. Each of those $k$ edges has a fixed weight: the first is 1, the second is 2, and so on up to $k$.

codeforcescompetitive-programmingdpimplementationtrees
LeetCode 2908 - Minimum Sum of Mountain Triplets I

The problem asks us to find the minimum possible sum of a "mountain triplet" within a given array of integers. A mountain triplet is defined as three indices (i, j, k) such that i < j < k, nums[i] < nums[j], and nums[k] < nums[j].

leetcodeeasyarray
LeetCode 2546 - Apply Bitwise Operations to Make Strings Equal

The problem provides two binary strings, s and target, of equal length n. You are allowed to perform a specific bitwise operation on s any number of times, which involves picking two distinct indices i and j and updating s[i] to s[i] OR s[j] and s[j] to s[i] XOR s[j].

leetcodemediumstringbit-manipulation
LeetCode 2443 - Sum of Number and Its Reverse

Here’s a complete, detailed solution guide for LeetCode 2443 - Sum of Number and Its Reverse, fully following your formatting rules.

leetcodemediummathenumeration
LeetCode 2122 - Recover the Original Array

The problem presents a scenario in which Alice has an original array arr of length n consisting of positive integers. She chooses a positive integer k and generates two new arrays: lower and higher.

leetcodehardarrayhash-tabletwo-pointerssortingenumeration
LeetCode 2562 - Find the Array Concatenation Value

The problem is asking us to compute a special sum over a given array nums by repeatedly concatenating the first and last elements of the array until it is empty.

leetcodeeasyarraytwo-pointerssimulation
CF 241A - Old Peykan

We are asked to model a journey along a straight line of cities connected by one-way roads, where a car travels at a constant speed of one kilometer per hour and consumes one liter of fuel per kilometer.

codeforcescompetitive-programminggreedy
LeetCode 2349 - Design a Number Container System

The problem asks us to design a data structure that supports two operations efficiently: 1. Assign a number to a given index. 2. Find the smallest index currently assigned to a given number.

leetcodemediumhash-tabledesignheap-(priority-queue)ordered-set
CF 186B - Growing Mushrooms

We have a mushroom-growing contest with two phases separated by a break. Each participant has two speeds, and the problem is that we do not know the order they will use them. During the first phase of length t1, mushrooms grow at one speed.

codeforcescompetitive-programminggreedysortings
LeetCode 2955 - Number of Same-End Substrings

The problem asks us to compute the number of same-end substrings within specified subranges of a given string s. A substring is same-end if its first and last character are identical. We are given multiple queries in the form [li, ri], each representing a substring s[li..ri].

leetcodemediumarrayhash-tablestringcountingprefix-sum
LeetCode 2367 - Number of Arithmetic Triplets

The problem requires us to find the number of arithmetic triplets in a strictly increasing array of integers. An arithmetic triplet (i, j, k) satisfies the conditions i < j < k, nums[j] - nums[i] == diff, and nums[k] - nums[j] == diff.

leetcodeeasyarrayhash-tabletwo-pointersenumeration
CF 161C - Abracadabra

The infinite string in this problem is built recursively. Start with: In general: The alphabet contains 36 symbols: The full string after 30 steps has length: $ For k = 30, the length is about 10^9, which matches the input bounds.

codeforcescompetitive-programmingdivide-and-conquer
LeetCode 2984 - Find Peak Calling Hours for Each City

The problem is asking us to analyze call records from a Calls table and determine the peak calling hour for each city. Each row in the table contains a callerid, recipientid, a timestamp (calltime), and the city where the call originated.

leetcodemediumdatabase
CF 149D - Coloring Brackets

We are given a valid parenthesis sequence. Every opening bracket has a unique matching closing bracket, and the pairs are properly nested. We want to assign colors to brackets under two rules.

codeforcescompetitive-programmingdp
LeetCode 2040 - Kth Smallest Product of Two Sorted Arrays

The problem asks for the k-th smallest product that can be formed by multiplying one element from the sorted array nums1 with one element from the sorted array nums2. Both arrays may contain negative numbers, zeros, and positive numbers.

leetcodehardarraybinary-search
CF 191B - Demonstration

We are asked to determine the earliest square where the opposition can hold a demonstration given the interference of the city administration. There are n squares arranged by increasing distance from the city center, with square 1 being the most central.

codeforcescompetitive-programminggreedy
LeetCode 2480 - Form a Chemical Bond

The problem asks us to identify all valid chemical bonds that can form between elements in a given table. The Elements table contains three columns: symbol, type, and electrons. The type column is an enumeration of 'Metal', 'Nonmetal', and 'Noble'.

leetcodeeasydatabase
LeetCode 2326 - Spiral Matrix IV

The problem gives us a linked list and asks us to place its values into an m x n matrix in clockwise spiral order. We begin filling from the top-left corner of the matrix, which is position (0, 0). The traversal direction follows the standard spiral pattern: 1.

leetcodemediumarraylinked-listmatrixsimulation