# Questions tagged [number]

A competition to solve a particular problem through the usage and manipulation of numbers.

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### Written Word Equation

Word equations, but not as you know it! Given a sentence which will include two numbers, numerically, and a spelt operator, in the order seen in the examples, your goal is to give the numerical answer ...
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Given a number $n ≥ 2$, a blackbox function $f$ that takes no arguments and returns a random integer in the range 0...n-1 inclusive, and a number $m ≥ n$, your challenge is to generate a random ...
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### Index and return 3 characters

The challenge is simple: Read 3 letters from a system input¹ and convert them to numbers (A-1, B-2, C-3...Z-26)². Print each of those numbers in the order the ...
378 views

### Will the hydra finally die? Part II

Background This is a follow up question to the question: Will the Hydra finally die? As before a dangerous A Medusa have released a dangerous Hydra which is revived unless the exact number of heads it ...
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### Ploughed fields via moduli

Let $R, C$ be positive integers and let $0 < s \leq 1$. Consider the $R \times C$ matrix $\mathbf M$ defined as \begin{equation} M(i,j) = \frac{\mathrm{mod}\,(j, i^s)}{R^s}, \quad i = 1, \...
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### Project Euler 1: Multiples in constant time

The purpose of this challenge is to solve the original first Project Euler problem, but as the title suggests in constant time (with respect to the size of the interval). Find the sum of all the ...
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### Is it a Giuga number?

Giuga numbers (A007850) are composite numbers $n$ such that, for each prime factor $p_i$ of $n$, $p_i \mid \left( \frac n {p_i} -1 \right)$. That is, that for each prime factor $p_i$, you ...
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### Will the Hydra finally die?

Background A Medusa have released a dangerous Hydra which is revived unless the exact number of heads it have is removed. The knights can remove a certain number of heads with each type of attack, and ...
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### Largest Number with No Repeating Digit Pairs

Inspired by the problem with the same name on Puzzling SE by our very own Dmitry Kamenetsky. You are to find the largest number that only uses every digit pair once, in a given base. For example, in ...
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### Mr. Binary Counterman

Mr. Binary Counterman, son of Mr. Boolean Masker & Mrs. Even Oddify, follows in his parents’ footsteps and has a peculiar way of keeping track of the digits. When given a list of booleans, he ...
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### Collatz's ice cream cone factory

The Collatz sequence Given a positive integer $a_1$, the Collatz sequence with starting value $a_1$ is defined as \begin{equation} a_{n+1} = \begin{cases} a_n/2 & \mathrm{if}\ a_n\ \mathrm{is}\...
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### A triangle of numbers

Challenge Make a triangle of numbers like below. How to? Let say we get a numbers in the form of ab, for example 12. We then create a triangle like below: Sorry ...
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### 'Plane' the numbers in any base!

Inspired by this Numberphile video. What is planing? In order to 'plane' a sequence of digits, you need to: Identify the lengths of the 'runs' (adjacently repeated digits) in the sequence. Non-...
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### Split a number in every way possible way within a threshold

Task Given a positive integer number and a threshold, split into every possible way without generating any numbers above the threshold. Examples Input: 15008 Threshold: 400 Valid Output: ...
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### Strobogrammatic Numbers [duplicate]

Strobogrammatic Numbers Definition A number which is rotationally symmetrical, i.e., it'll appear the same when rotated by 180 deg in the plane of your screen. The following figure illustrates it ...
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### Possible binaries sum

This challenge is inspired by the AoC 2020, Day 14 - Part II, created by Eric Wastl and his team, which asks to output the possible binary values from a bitmask. Let's say we've a bitmask like ...
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### Towering Cistercian representation

SE Sandbox Link, Codidact Sandbox Link Inspired by this video. Given a positive integer, draw its Cistercian representation as ascii art. The Challenge Cistercian numerals are a decimal-based number ...
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### Convert negadecimal to decimal (and back)

Negadecimal, also known as base -10, is a non-standard positional numeral system. Take the number $1337_{10}$. In decimal, this has the value one thousand three hundred thirty seven, and can be ...
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### Fastest square root of an arbitrary size

We do seem to have a fastest square root challenge, but it's very restrictive. In this challenge, your program (or function) will be given an arbitrarily sized nonnegative integer, which is the square ...
Inspiration Given a positive integer $1 \le n \le 9$, output all positive $n$-digit integers $i$ for which the following is true: Each digit from $1$ to $n$ appears exactly once in $i$. ...