Stack Exchange Network

Stack Exchange network consists of 174 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Visit Stack Exchange

Questions tagged [number-theory]

Number theory involves properties and relationships of numbers, primarily positive integers.

-3
votes
0answers
141 views

Permutations to the nines redux [on hold]

Note: This question has been challenging to describe; full disclosure How to make question clear to users who voted to close without providing feedback as to what is not clear to them?; OEIS A217626: ...
14
votes
6answers
1k views

Ryley's Theorem

S. Ryley proved following theorem in 1825: Every rational number can be expressed as a sum of three rational cubes. Challenge Given some rational number \$r \in \mathbb Q \$ find three rational ...
15
votes
11answers
845 views

Generate some rough numbers

Background A number n can be described as B-rough if all of the prime factors of n strictly ...
19
votes
10answers
2k views

Repeat this GCD operation

Problem A3 from the 2008 Putnam competition says: Start with a finite sequence \$a_1, a_2, \dots, a_n\$ of positive integers. If possible, choose two indices \$j < k\$ such that \$a_j\$ does not ...
18
votes
26answers
3k views

Is the word coprime?

Given a word, treat every letter as it's number in English alphabet (so a becomes 1, b becomes 2, ...
13
votes
10answers
1k views

Quadratic residues are so much fun!

Definitions Quadratic residues An integer \$r\$ is called a quadratic residue modulo \$n\$ if there exists an integer \$x\$ such that: $$x^2\equiv r \pmod n$$ The set of quadratic residues modulo \...
14
votes
2answers
360 views

Impatient divisibility test

Your task is to write a program or function that determines whether a number is divisible by another. The catch is that it should give an answer as soon as possible, even if not all digits of the ...
13
votes
7answers
637 views

Make the biggest and smallest numbers

Inspired by this post over on Puzzling. Spoilers for that puzzle are below. Given three positive integers as input, (x, y, z), construct the inclusive range ...
15
votes
16answers
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 (...
12
votes
17answers
484 views

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'\$ ...
17
votes
7answers
591 views

Split the bits!

We define \$V(x)\$ as the list of distinct powers of \$2\$ that sum to \$x\$. For instance, \$V(35)=[32,2,1]\$. By convention, powers are sorted here from highest to lowest. But it does not affect ...
28
votes
77answers
13k views

Is this number evil?

Introduction In number theory, a number is considered evil if there are an even number of 1's in its binary representation. In today's challenge, you will be identifying whether or not a given number ...
24
votes
13answers
3k views

Find the 10-adic cube root of 3

I like to think of a 10-adic number as a number that goes infinitely to the left, or an integer modulo a very very large power of 10. Things carry infinitely to the left and vanish. To see what I ...
19
votes
16answers
2k views

Is this a consecutive-prime/constant-exponent number?

A while ago, I had a look at the prime factorization of 27000: 27000 = 23 × 33 × 53 There are two special things about that: consecutive-prime: The primes are consecutive: 2 is the ...
27
votes
24answers
2k views

The Add-Multiply-Add Sequence

(Related) Given an integer n > 1, 1) Construct the range of numbers n, n-1, n-2, ... 3, 2, 1 and calculate the sum 2) Take ...
51
votes
11answers
3k views

Bringing a pair of integers to equality

This was inspired by a math problem I saw somewhere on the internet but do not remember where (UPDATE: The original problem was found on the math riddles subreddit with a proof provided that it is ...
11
votes
11answers
1k views

Am I a Pillai prime?

A Pillai prime is a prime number \$p\$ for which there exists some positive \$m\$ such that \$(m! + 1) \equiv 0 \:(\text{mod } p)\$ and \$p \not\equiv 1\:(\text{mod }m)\$. In other words, an ...
7
votes
2answers
189 views

Minimal Triangles

Make an upside down triangle of positive integers. Every number in the triangle must be distinct. Each number is the summation of its two parents (similar to how Pascal's triangle is constructed, but ...
22
votes
11answers
696 views

Digit Product Sequences

Here's an interesting sequence discovered by Paul Loomis, a mathematician at Bloomsburg University. From his page on this sequence: Define ...
5
votes
27answers
1k views

Simple Factorial Challenge [duplicate]

In light of today's date... A factorial of a number n, is the product of all the numbers from 1 to n inclusive. The Challenge Given an integer n where 0 <= n <= 420, find the sum of the ...
18
votes
13answers
2k views

Conway's Prime Game

Specifically, Conway's PRIMEGAME. This is an algorithm devised by John H. Conway to generate primes using a sequence of 14 rational numbers: ...
14
votes
9answers
1k views

Seidel Triangle

The Seidel Triangle is a mathematical construction similar to Pascal's Triangle, and is known for it's connection to the Bernoulli numbers. The first few rows are: ...
9
votes
5answers
359 views

Four Spiraling Axes

Take the numbers 0, 1, 2, 3, 4, ... and arrange them in a clockwise spiral, starting downward, writing each digit in its own separate square. Then, given one of ...
17
votes
26answers
1k views

Rotational Average

Given an input integer n >= 10, output the average of all deduplicated rotations of the integer. For example, for input 123, ...
27
votes
4answers
1k views

1, 2, 3, 14… or is it 15?

A well known song by the Irish rock band U2 starts with the singer Bono saying "1, 2, 3, 14" in Spanish ("uno, dos, tres, catorce"). There are various theories as to the significance of those numbers....
3
votes
3answers
186 views

unRSA: solve the private key

Given positive integer n and e, knowing that e<n and that ...
19
votes
3answers
365 views

Swap program halves to test divisors

Four integer sequences In this challenge, you will test four different properties of a positive integer, given by the following sequences. A positive integer N is perfect (OEIS A000396), if the sum ...
11
votes
10answers
973 views

Some Lonely Primes

I know, I know, yet another primes challenge... Related A lonely (or isolated) prime is a prime number p such that p-2, ...
20
votes
14answers
1k views

Is this a truncated triangular number?

Related OEIS sequence: A008867 Truncated triangular number A common property of triangular numbers is that they can be arranged in a triangle. For instance, take 21 and arrange into a triangle of <...
11
votes
3answers
819 views

Crazy but Rational Bases

We have many challenges based on base 10, base 2, base 36, or even base -10, but what about all the other rational bases? Task Given an integer in base 10 and a rational base, return the integer in ...
11
votes
5answers
614 views

Sparse Protractor

Given some positive integer n, design a protractor with the fewest number of marks that lets you measure all angles that are an integral multiple of ...
31
votes
10answers
1k views

Find the dot product of Rationals

I was at a friend's house for dinner and they suggested the idea of a "Prime-factor vector space". In this space the positive integers are expressed as a vector such that the nth element in the ...
10
votes
4answers
264 views

Continued Fraction of Digit-wise Sum of Square Roots

Introduction Your task is to generate the first 1000 terms in the continued fraction representation of digit-wise sum of square root of 2 and square root of 3. In other words, produce exactly the ...
14
votes
1answer
465 views

Regex for multiples of 9

It is easy to describe a finite state machine that recognizes multiples of 9: keep track of the digit sum (mod 9) and add whatever digit is accepted next. Such a FSM has only 9 states, very simple! By ...
-4
votes
1answer
126 views

Find prime factors of sum of non-composite Fibonacci numbers up to n

The Challenge Given a number, find the sum of the non-composite numbers in the Fibonacci sequence up to that number, and find the prime factors of the sum. For example, if you were given 8, the non-...
22
votes
20answers
3k views

Bertrand's Primes

Bertrand's Postulate states that for every integer n ≥ 1 there is at least one prime p such that n < p ≤ 2n. In order to verify this theorem for n < 4000 ...
14
votes
9answers
668 views

Verify Cyclic Difference Sets

A cyclic difference set is a set of positive integers with a unique property: Let n be the largest integer in the set. Let r be ...
23
votes
29answers
2k views

Pascal's Triangle (Sort of)

Most everyone here is familiar with Pascal's Triangle. It's formed by successive rows, where each element is the sum of its two upper-left and upper-right neighbors. Here are the first ...
15
votes
15answers
1k views

The modulo parity party

You are given an array A of n strictly positive integers, with n ≥ 2. Your task is to map each entry Ai to: 1 if Aj mod Ai is odd for each j such that 1 ≤ j ≤ n and j ≠ i 2 if Aj mod Ai is even for ...
15
votes
19answers
1k views

Make Zero From First 'n' Numbers

Challenge The challenge is to write a code that takes a positive integer 'n' as an input and displays all the possible ways in which the numbers from 1 - n can be written, with either positive or ...
32
votes
8answers
799 views

Can square tree rings be generated from primes?

Apparently yes! In three easy steps. Step 1 Let f(n) denote the prime-counting function (number of primes less than or equal to n). Define the integer sequence s(n) as follows. For each positive ...
26
votes
22answers
2k views

Diluted Integer Sums

A positive integer can be diluted by inserting a 0 between two bits in its binary expansion. This means that an n-bit number has ...
24
votes
12answers
2k views

Write numbers as a difference of Nth powers

Challenge There are many numbers which can be expressed as the difference of two squares, or as the difference of two cubes, or maybe even higher powers. Talking about squares, there are various ways ...
12
votes
14answers
1k views

Minimise the count of prime factors through insertion

Given two positive integers A and B, return the position p that minimises the number of prime factors (counting multiplicities) of the resulting integer, when B is inserted in A at p. For example, ...
8
votes
20answers
977 views

Prime numbers between n and 2n [duplicate]

Bertrand's postulate states that there is always at least 1 prime number between n and 2n for all n greater than 1. Challenge Your task is to take a positive integer n greater than 1 and find all of ...
21
votes
16answers
2k views

Recursive Collatz Conjecture

The Collatz conjecture postulates that if you take any positive integer, then repeat the following algorithm enough times: ...
16
votes
33answers
2k views

Find the largest number of distinct integers that sum to n

The Task Given an input positive integer n (from 1 to your language's limit, inclusively), return or output the maximum number of distinct positive integers that ...
11
votes
1answer
444 views

Write it into number theory style

Write a mathematical statement, using the symbols: There exists at least one non-negative integer (written as E, existential ...
20
votes
16answers
2k views

Factor-poor numbers

If a positive integer \$N > 2\$ has (strictly) less prime factors (without counting multiplicities) than its successor and its predecessor, we will call it a factor-poor number. In other words, \$...
20
votes
20answers
1k views

Determine Superabundance

A superabundant number is an integer n that sets a new upper bound for its ratio with the divisor sum function σ. In other words, n is superabundant if and only if, for all positive integers x ...