4
Based on this Math.SE question; number copied from this answer. Number originally from a Numberphile video, of course.
Your task is to output the following 1350-digit prime number:
888888888888888888888888888888888888888888888888888888888888888888888888888888888888888888888111111111111111111111...
Ethiopian Multiplication
This question is inspired by this answer. Turned out I used to use Ethiopian Multiplication when I was a kid, but had never known the name of the method until recently.
Ethiopian multiplication is a method of multiplying integers using only addition, doubling, and halvi...
The "Code Golf Ratio"
code-golfstack-exchange-api
Don't you ever want to know if your code golf score is "good enough" for the language your using? How can you measure which golf is better when one is in Jelly and one is in JS? Well, now we can!
Task
Take two programming language names as inp...
My job is stacking pebbles into triangular piles. I've only been doing this for a century and it is already pretty boring. The worst part is that I label every pile. I know how to decompose pebbles into piles of maximal size, but I want to minimize the number of piles. Can you help?
Task
Given ...
In the game of Freecell, you are tasked with building four foundation piles in suit from ace to king, on a layout where you build downward in alternating colours. However, you can only build one card at a time, so you are given four "free cells" each of which can contain one card to help you move...
Based on this challenge and this Math.SE question. Idea originally come from a Numberphile video. The goal is to reproduce the logic of Professor McKee when he builds this prime.
Your task is to build the Trinity Hall Prime, at a much lower scale. You are given a single number N greater than 2, ...
kolmogorov-complexity
As some kind of tribute to those affected by Harvey, Irma, and other hurricanes.
Input
Your program must support one (or both) of these input modes:
An integer n, ranging from 0 to 20 (or 1 to 21 if you like to have a one-based solution)
A single character c. You may ch...
This is how the Kolakoski sequence (OEIS A000002) is defined:
The Kolakoski sequence is a sequence that contains 1 and 2, and the nth element of the sequence is the length of the nth group of equal elements (run) in the sequence itself. The first 20 terms of the sequence and the respective le...
You know that task from IQ tests: You are given a sequence of 5 integers <1000, and must "guess" the sixth.
E.g.
1,2,3,4,5 : 6
110,100,90,80,70: 60
2,4,8,16,32: 64
1,17,8,99,23: NaN
The algorithm is either "add a fixed number to the last one" (first two examples), or "multiply the last numbe...
Based on a chat message
The Challenge
Given an input number n > 9, construct its reverse, ignoring leading zeros. Then, construct a list of all prime factors that the number and its reverse don't have in common. Multiply those factors together to create the Uncommon Factor Number of the input.
...
Most challenges that take a true/false or right/left or something like that input allow you to pick any two distinct values for these inputs. Recently, this answer came up. Clever use of this specification; however, I feel like this is cheating in some sense, and, as AdmBorkBork points out, it is...
Erdős–Straus conjecture
This is a conjecture proposed by Paul Erdős and Ernst N. Straus where it simply defines for any x >= 2, then:
4 1 1 1
--- = --- + --- + ---
x a b c
where a, b, and c are whole numbers (positive integers).
For example, if x was 5 then:
4 ...
