78 questions linked to/from Is this number a prime?
2k views

### Prime or highest factor

Challenge: Given an array of non-negative whole numbers numbers in the range of 0 to Infinity, Check whether all of them are primes or not. (You can take input as ...
2k views

### Additive Primes amongst first x Primes

Definition of Additive Primes: Numbers which have exactly 2 divisors are called Prime numbers. Numbers which are prime and their sum of digits is also a prime number are called Additive Primes Task:...
• 42.4k
3k views

### Create a programming language that only appears to be unusable (Robbers' thread)

See the cop thread for more information. Each answer to this question should crack an answer there. That is to say, it should be code to find the third-largest integer in the input when run in the ...
• 31.3k
2k views

### Array Escape - get out of there

One day you awake only to find yourself caught in an array. You try to just walk out of there, taking one index at the time, but it seems there are other rules: The array is completely filled with ...
3k views

### ​Cuban​ ​Primes

Given a natural number $n$, return the $n$-th cuban prime. Cuban Primes A cuban prime is a prime number of the form $$p = \frac{x^3-y^3}{x-y}$$ where $y>0$ and $x = 1+y$ or $x = 2+y$ ...
• 43.8k
997 views

### Print the missing primes

The Task Write a program or function that, when passed a numerical input x, prints or returns the primes beneath the square root of ...
• 11.2k
3k views

### Encode an integer

Given positive integer n > 2. We convert it to an array as follows: If it is equal to 2 return an empty array Otherwise ...
1 vote
2k views

### Is it a completely even number?

A completely even number is a positive even integer whose divisors (not including 1) are all even. Some completely even numbers are: 2 (divisors: 2, 1) 4 (divisors: 4, 2, 1) 16 (divisors: 16, 8, 4, 2,...
• 13.8k
1k views

### Greatest Number in a Range When The Sum of Squares of its Prime Factors are Subtracted

The formula Take for instance the number 300 The prime factors of 300 are [2, 3, 5] (unique numbers that are factors of 300 and prime) Squaring each of those ...
• 491
3k views

### It almost works (Robbers' Thread)

Cops' Thread Your challenge is to find an uncracked, unsafe cop on the cops thread, and find an input value for which their code does not do as specified by the challenge. You must use values within ...
• 37.3k
979 views

### Print/Output all positive numbers in which every multi-digit substring in its decimal representation is also prime.

Task Your task is to print or output all positive numbers in which every multi-digit substring in its decimal representation is also prime. If the number has at least 2 digits, this would imply that ...
• 49.7k
2k views

### The number of ways a number is a sum of consecutive primes

Given an integer greater than 1, output the number of ways it can be expressed as the sum of one or more consecutive primes. Order of summands doesn't matter. A sum can consist of a single number (...
• 4,304
2k views

### Prime Time Travel

Don't tell anyone, but I've nicked my uncle's time travel machine! My uncle is obsessed with prime numbers, though, and that shows in the machine — he has programmed it so that it can only go to dates ...
• 6,612
2k views

### Recognize most smaller primes with a regex

This time, you are working on a regex. Your regex is meant to approximately full-match the base-10 representations of primes $0 \le p < 1000$, while ignoring any non-numeric string or composite ...
• 3,839
### Compute the minimum $a(n)>a(n-1)$ such that $a(1)+a(2)+\dots+a(n)$ is prime (OEIS A051935)
Background Consider the following sequence (A051935 in OEIS): Start with the term $2$. Find the lowest integer $n$ greater than $2$ such that $2+n$ is prime. Find the lowest integer $n'$ ...