Questions tagged [number-theory]

number theory is the branch of mathematics concerning the mathematical properties of numbers and the relationships between various types of numbers. This tag should be used with questions regarding computer science topics which are presented from a number theory perspective or may involve number theory or whose answer could be or should be couched in number theory terms.

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23 views

Given a set of integers $D$ and a positive value$P$, find an algorithm to find set of integers satisfying a condition

Given a set of positive integers : $ \\ D = \{ D_1, D_2, ..., D_n\}$ and a non-negative integer $P$, where $P$ is divisible by every element in $D$, then find ...
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0answers
57 views

Counting triplets from three arrays satisfying the equation x^2 = yz

Let's say I have three arrays of positive integers X, Y and Z. You can assume that each of ...
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0answers
19 views

Circuit depth of computing the continued fractions of a rational number

If you want to convert a rational number into its continued fraction, what is the circuit depth of this process, in terms of the total number of bits of input? I was reading through some notes which ...
3
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1answer
53 views

Efficiently prime factorising an integer with an oracle

Suppose you have a program one_factor(N) that, given an n-digit binary number, N, returns ...
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1answer
68 views

Finding a winner in a “long list”

This is from betting domain which has something that is called a long list: a list of a "home team win/draw/away team win" markets for 13 games. A punter can select any combination of the possible ...
3
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1answer
58 views

Number of ways n can be written as sum of at least two positive integers

I found a solution in Python for this problem, but do not understand it. The problem is how many ways an integer n can be written as the sum of at least two positive integers. For example, take n = 5. ...
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1answer
37 views

What is the time complexity of determining whether a solution $x$ exists to $x^k \equiv c \pmod{N}$ if we know the factorization of $N$?

Suppose we are given an integer $c$ and positive integers $k, N$, with no further assumptions on relationships between these numbers. We are also given the prime factorization of $N$. These inputs are ...
3
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0answers
41 views

Quick calculation for $x^y \bmod 2^d$

I need to calculate $x^y \bmod 2^d$ in $O(d)$ summations/bitwise operations and $1$ multiplication by $y$. $x$ is restricted to be odd, $d\geq 3$. $a$-bit arithmetic (for any $a$) is allowed, as this ...
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0answers
47 views

Branch and scope globally unique identifiers

Say we are working with a Prolog-like system where variables are dynamically created in different branch contexts and scopes, yet these variables are also globally viewable by the system regardless of ...
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0answers
21 views

Prove that x and y in extended Euclid's algorithm won't overflow an Integer (If a,b <= 1e8, ax+by=gcd(a,b))

We are given a and b <= 1e8. The extended Euclid's algorithm always finds a solution for ax+by=gcd(a,b) (assuming it exists) which can always be stored in an Int. How to prove the x and y won't ...
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1answer
40 views

Prove, a^2+b^2=c^2,there exists only 1 case such that a,b,c are consecutive non negative integers(3,4,5) [closed]

I want to prove, $a^2+b^2=c^2$,there exists only 1 case such that a,b,c are consecutive non-negative integers(3,4,5). I have no clue to prove this lemma. Please help me to prove this lemma.
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81 views

Given $n=pq=a^2+b^2$, can we factor $n$?

Just to be clear, $a$ and $b$ are known, while $p$ and $q$ are unknown prime numbers, both congruent to $1$ modulo $4$. Can we design an efficient algorithm to retrieve $p$ and $q$? It is a known ...
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1answer
55 views

Guess the number from its different base representations

Given a set of numbers in different representations (we don't know the value of the base in which we are representing) of bases, find the original number (in decimal representation) if it exists or ...
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1answer
99 views

How to Optimise the following algorithm? maximum good value of an element in an array

I was recently stuck while doing a question, please suggest a way with a code/pseudo code to optimize the following algorithm for finding the maximum good value in an array where a good value of an ...
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1answer
30 views

Simple generator of pseudo-random permutations of variable length short sequence

The problem in front of me is to write a function (from scratch) to permute n elements, where n is an argument. I decided to break it down to applying Knuth's shuffles algorithm, therefore I needed to ...
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1answer
23 views

Use Fact to decompose a given number

Let Fact be the following decision problem. Given two natural numbers $k \le n$ the machine accepts if and only if $n$ has a non-trivial divisor that is less than or equal to $k$. Prove that if Fact $\...
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1answer
57 views

How to count all integers less than a given integer and having two contigous digits as $y$?

Suppose i have been given a number 54432 .How to count all numbers less than 54432 and having last two digits as 1 ? i.e all the numbers of form xxx11 and xxx11 < 54432 .Here x can be any digits ...
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1answer
51 views

How do I minimize the cost of some algorithm that performs some operation on a list?

I stumbled upon this problem whilst studying the complexity of a simple algorithm. I used set-theoretic notation, but all the $S_i$'s are lists (I couldn't think of a better way to write the problem ...
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1answer
60 views

Express a number with a combination of arithmetic operations and a single number

Today I ran into a this relatively simple (At least from my perspective ) problem. Basically the task is to be able to express any number using only a single number an any combination of arithmetic ...
3
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2answers
95 views

Proving that a set of operations can't generate one integer from a given one

Given two numbers, $n$ and $m$, are there some mathematical methods of deducing $m$ from $n$ using limited number of elemantary operations? Example: 335 can be deduced from 2000 using division by 2, ...
3
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4answers
141 views

How to select best k fractions out of n fractions (k<=n) so as to have (numerator sum / denominator sum) maximum?

For example, given 4 fractions $\frac{4}{2}$, $\frac{2}{3}$, $\frac{1}{2}$, $\frac{10}{20}$, I have to select 3 fractions out of these 4 so that the value of $\frac{\text{numerator sum}}{\text{...
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1answer
51 views

I want to find the number of steps it takes to find the GCD by Euclidean Algorithm

Let's say I have two numbers a and b. I want to find the number of steps it takes to find the GCD by Euclidean Algorithm by a closed formula which includes parameter a and b. If I go by this ...
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1answer
32 views

Coprimes satisfying a pair

We know that number of coprimes less than a number can be found using Euler's totient function. But if there are two numbers $p$ and $q$ and we need to find number of numbers less than $q$ ...
3
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2answers
131 views

How does the bitlength of the divisor affect the running-time complexity of division algorithms?

Wikipedia lists $O(M(n))$ as the best complexity (out of the algorithms listed) for division on two $n$-digit numbers, where $M(n)$ is the complexity of the multiplication algorithm of choice. This is ...
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2answers
43 views

Running time complexity of finding maximal power of divisor that divides natural number

Given $n \in \mathbb{N}$, a divisor $p\vert n$, I would like to efficiently find $e\in\mathbb{N}$ with $p^e \vert n$, and $e$ maximal with this property. I will assume that multiplication/division of ...
2
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1answer
68 views

Generalizing Knuth's $O(\log_2 n)$ Fibonacci algorithm to linear homogenous recurrences

Knuth has a neat algorithm that uses matrix exponentiation to compute the $n$th Fibonacci number in $O(\log_2 n)$-time 1. However, there doesn't seem to be a lot of resources on generalizing his idea ...
2
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1answer
71 views

Bertrand's ballot theorem

I want to understand the dynamic programming equation of https://en.wikipedia.org/wiki/Bertrand%27s_ballot_theorem theorem. it is this If i number of people voted for A and j number of people voted ...
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1answer
148 views

Carpet into Box

Given a carpet of size a * b [length * breadth] and a box of size c * d, one has to fit the carpet in the box in the minimum number of moves. A move is to fold the carpet in half, either by length or ...
2
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2answers
27 views

An approximate quantity of multiplications in $\mathbb{F}_p$ amounting the same bit complexity as one inversion in $\mathbb{F}_p$

Consider a prime finite field $\mathbb{F}_p$ of quite large characteristics $p$, for example $\log_2(p) \approx 256$ bits. I would like to know an approximate quantity of multiplications in $\mathbb{F}...
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1answer
66 views

Number Theory Problem from Local Selection Contest EPFL | ETHZ

This was a question from the 2016 local (selection) contest in ETHZ, You have a high-precision alarm clock with three operations: 1) reset wake-up time to midnight (00:00:00.000000) 2) modify the ...
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2answers
95 views

Algorithm for generate all solutions to a linear Diophantine equation

Consider the linear Diophantine equation of the form: $$\sum_{i=1}^{k}a_ix_i=n.$$ My goal is to list all the non-negative solutions to this equation. I wrote the following recursive algorithm, but I ...
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1answer
109 views

Modular exponentiation in P

I need to prove that the following language is in $\mathsf{P}$: $$ L = \{\langle x,y,z,d \rangle : xˆy \equiv z \pmod{d}\}.$$ I'm assuming I just have to prove with an algorithm or negate that it's ...
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0answers
22 views

Is finding a sum of two squares representation for a number computationally assymptotically equivalent to integer factorization?

I have been wondering this question because given a number $n$ with prime factors of the form $4k+3$ when we square $n$ and find a sum of two squares which should reveal these types of factors (as ...
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1answer
121 views

Checking Divisibility Using Minimal Bits

Suppose we're given an infinite stream of integers, $x_1, x_2, \dots$. a) Show that we can compute whether the sum of all integers seen so far is divisible by some fixed integer $N$ using $O(\log ...
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2answers
166 views

Randomly Choosing a N-Bit Prime

I've been studying some number theory, and I came across this problem: Lagrange’s prime number theorem states that as N increases, the number of primes less than $N$ is $Θ(N/ log(N))$. ...
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1answer
31 views

How to calculate $\sum_{i=1}^n \mu^2(i)$ in less than $O(n)$'s time

To go with $O(n)$, we can use the linear sieve according to that $\mu(n)$ is multiplicative. But it seems that we don't have to work each $\mu(n)$ out and accumulate them together, because I only want ...
3
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0answers
16 views

Base-k representations of polynomials: state of art [closed]

In chapter 4 of Jeffrey Shallit's A Second Course in Automata Theory the following problem is formulated as open: Let $p(n)$ be a polynomial with rational coefficients such that $p(n) \in \mathbb{N}$ ...
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1answer
36 views

Two's complement max with a different base

Working with fixed size integer representations, use a number system with b-complement notation, base b = 9 and n = 4 digits. What is the smallest number that can be represented in this number system?...
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3answers
73 views

Determining number of elements divisible by at least one prime of set

I have an array ($|A|\leq 10^6$) of numbers ($A_i\leq10^6$) and a set of prime numbers. I have to find the count of the elements in the array that are divisible by at least one of the numbers in the ...
2
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1answer
86 views

How to estimate the average time complexity of greatest common divisor?

As we know, the time complexity of $\gcd(x,y)$ is $O(\log \min(x,y))$ by using Euclidean algorithm. Now we fix a constant $n$ and consider the average time complexity of $\gcd(x,n)$. Formally, let $f(...
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1answer
74 views

Maximum trailing zeros of the path

Problems: A table with $n$ rows and $m$ columns is filled with number from $1$ to $100$ (duplication allowed). The player starts at $(1, 1)$. He can only move right or down. The goal is to reach $(n, ...
4
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2answers
75 views

Count numbers less than $x$ co-prime to $p$

We have given two numbers $x$ and $p$. We want to count how many numbers are less than $x$ and are co-prime with $p$. I know that we can solve the problem in $O(x\log x)$ with iterating over all ...
4
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2answers
199 views

Which is the fastest method for calculating exact square root of a integer of 200-500 digit number?

I wanted to know is there any algorithm / function / process through which I can calculate square root of a very large integer number. I wants to know current state of the research in this field. No ...
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1answer
129 views

What is the most compact (space efficient) method of storing an array of distinct integers?

I have an array of distinct integers which I want to save in the most compact manner. I may have to do occasional lookups, deletions, and insertions in this array so the compression algorithm must ...
4
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0answers
109 views

Optimal parallel-time repeated modular squaring circuit

Given a 4096-bit integer $x$ and a 4096-bit RSA modulus $N$ (of unknown factorisation) what is the fastest circuit to compute $x^{2^T} \mod N$ where $T=2^{40}$. That is, what is the fastest parallel-...
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1answer
35 views

How hexadecimal representation is more compact and intelligible for documentation?

My textbook says, "Instead, it is far better to use a hexadecimal representation for documentation purposes. Whether or not a code represents a binary number, it can be treated as ...
6
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0answers
39 views

Level sums, displacements: how to determine their effect efficiently?

Let $R =\mathbb{Z}/N \mathbb{Z}$. Let $f:R\to \mathbb{R}$, $\rho:R\to \lbrack 0,1\rbrack$. We assume that it takes trivial time to compute any given value $f(m)$ or $\rho(m)$. Define $$S(\delta,m) = ...
6
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0answers
225 views

Testing algorithm for a modified sieve of Eratosthenes

Context: I am looking at a modified version of the sieve of Eratosthenes. I started by generalising Eratosthenes' sieve, like so: Choose some starting "root", $n_0\in\mathbb{N}$, a sieve limit (the ...
4
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2answers
82 views

Proving that $\{0^{m^2}\mid m\geq 3\}^*$ is regular

We know that $L=\{0^{m^2}\mid m\geq 3 \}$ is not a regular language. However $L^*$ is regular because we can generate $0^{120}$ to $0^{128}$ by some concatenations and then any other power of $0$ can ...
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1answer
332 views

Symmetric difference of a set with an empty set [closed]

The definition of symmetric difference of two sets $\alpha $ and $\beta$, $\alpha \oplus \beta$ is defined as the set of all $x$ such that, $x \in (\alpha \cup \beta) - (\alpha \cap \beta)$. If, $\...