Questions based on various concepts of number theory and different types of number are quite frequently asked in programming contests. In this article, we discuss some famous facts and algorithms:

**Interesting Facts :**

- All 4 digit palindromic numbers are divisible by 11.
- If we repeat a three-digit number twice, to form a six-digit number. The result will will be divisible by 7, 11 and 13, and dividing by all three will give your original three-digit number.
- A number of form 2
^{N}has exactly N+1 divisors. For example 4 has 3 divisors, 1, 2 and 4. - To calculate sum of factors of a number, we can find the number of prime factors and their exponents. Let p
_{1}, p_{2}, … p_{k}be prime factors of n. Let a_{1}, a_{2}, .. a_{k}be highest powers of p_{1}, p_{2}, .. p_{k}respectively that divide n, i.e., we can write n as**n = (p**._{1}^{a1})*(p_{2}^{a2})* … (p_{k}^{ak})Sum of divisors = (1 + p

_{1}+ p_{1}^{2}... p_{1}^{a1}) * (1 + p_{2}+ p_{2}^{2}... p_{2}^{a2}) * ............................................. (1 + p_{k}+ p_{k}^{2}... p_{k}^{ak}) We can notice that individual terms of above formula are Geometric Progressions (GP). We can rewrite the formula as. Sum of divisors = (p_{1}^{a1+1}- 1)/(p_{1}-1) * (p_{2}^{a2+1}- 1)/(p_{2}-1) * .................................. (p_{k}^{ak+1}- 1)/(p_{k}-1) - For a product of N numbers, if we have to subtract a constant K such that the product gets its maximum value, then subtract it from a largest value such that largest value-k is greater than 0. If we have to subtract a constant K such that the product gets its minimum value, then subtract it from the smallest value where smallest value-k should be greater than 0
- Goldbech`s conjecture: Every even integer greater than 2 can be expressed as the sum of 2 primes.
- Perfect numbers or Amicable numbers: Perfect numbers are those numbers which are equal to the sum of their proper divisors. Example: 6 = 1 + 2 + 3
- Lychrel numbers: Are those numbers that cannot form a palindrome when repeatedly reversed and added to itself. For example 47 is not a Lychrel Number as 47 + 74 = 121
- Lemoine’s Conjecture : Any odd integer greater than 5 can be expressed as a sum of an odd prime (all primes other than 2 are odd) and an even semiprime. A semiprime number is a product of two prime numbers. This is called Lemoine’s conjecture.
- Fermat’s Last Theorem : According to the theorem, no three positive integers a, b, c satisfy the equation, for any integer value of n greater than 2. For n = 1 and n = 2, the equation have infinitely many solutions.

**Number Theory Algorithms**

**GCD and LCM**

Recent Articles on GCD and LCM!

**Prime Factorization and Divisors :**

- Prime factors
- Pollard’s Rho Algorithm for Prime Factorization
- Find all divisors of a natural number
- Sum of all proper divisors of a natural number
- Prime Factorization using Sieve O(log n) for multiple queries
- Find politeness of a number
- Print prime numbers in a given range using C++ STL
- k-th prime factor of a given number
- Smith Numbers

Recent Articles on Prime Factors!

**Fibonacci Numbers:**

- Fibonacci Numbers
- Interesting facts about Fibonacci numbers
- How to check if a given number is Fibonacci number?
- Zeckendorf’s Theorem (Non-Neighbouring Fibonacci Representation)

Recent Articles on Fibonacci Numbers!

**Catalan Numbers :**

Recent Articles on Catalan Numbers!

**Modular Arithmetic :**

- Modular Exponentiation (Power in Modular Arithmetic)
- Modular multiplicative inverse
- Modular Division
- Multiplicative order
- Find Square Root under Modulo p | Set 1 (When p is in form of 4*i + 3)
- Find Square Root under Modulo p | Set 2 (Shanks Tonelli algorithm)
- Euler’s criterion (Check if square root under modulo p exists)
- Multiply large integers under large modulo
- Find sum of modulo K of first N natural number
- How to compute mod of a big number?
- BigInteger Class in Java
- Modulo 10^9+7 (1000000007)
- How to avoid overflow in modular multiplication?
- RSA Algorithm in Cryptography
- Find (a^b)%m where ‘a’ is very large
- Find power of power under mod of a prime

Recent Articles on Modular Arithmetic!

- Euler’s Totient Function
- Optimized Euler Totient Function for Multiple Evaluations
- Euler’s Totient function for all numbers smaller than or equal to n
- Primitive root of a prime number n modulo n

**nCr Computations :**

- Binomial Coefficient
- Compute nCr % p | Set 1 (Introduction and Dynamic Programming Solution)
- Compute nCr % p | Set 2 (Lucas Theorem)
- Compute nCr % p | Set 3 (Using Fermat Little Theorem)

**Chinese Remainder Theorem :**

- Set 1 (Introduction)
- Set 2 (Inverse Modulo based Implementation)
- Cyclic Redundancy Check and Modulo-2 Division
- Using Chinese Remainder Theorem to Combine Modular equations

**Factorial :**

- Factorial
- Legendre’s formula (Given p and n, find the largest x such that p^x divides n!)
- Sum of divisors of factorial of a number
- Count Divisors of Factorial
- Compute n! under modulo p
- Recent Articles on Factorial!
- Wilson’s Theorem
- Primality Test | Set 1 (Introduction and School Method)
- Primality Test | Set 2 (Fermat Method)
- Primality Test | Set 3 (Miller–Rabin)
- Primality Test | Set 4 (Solovay-Strassen)
- GFact 22 | (2^x + 1 and Prime)
- Euclid’s Lemma
- Sieve of Eratosthenes
- Segmented Sieve
- Sieve of Atkin
- Sieve of Sundaram to print all primes smaller than n
- Sieve of Eratosthenes in 0(n) time complexity
- Check if a large number is divisible by 3 or not
- Check if a large number is divisible by 11 or not
- To check divisibility of any large number by 999
- Carmichael Numbers
- Generators of finite cyclic group under addition
- Measure one litre using two vessels and infinite water supply
- Program to find last digit of n’th Fibonnaci Number
- GCD of two numbers when one of them can be very large
- Find Last Digit Of a^b for Large Numbers
- Remainder with 7 for large numbers
- Count all sub-arrays having sum divisible by k
- Partition a number into two divisible parts
- Number of substrings divisible by 6 in a string of integers
- ‘Practice Problems’ on Modular Arithmetic
- ‘Practice Problems’ on Number Theory
- Ask a Question on Number theory
- Padovan, OESIS

**Prime numbers and Primality Tests :**Recent Articles on Prime Number!

**Sieve Algorithms :****Divisibility and Large Numbers :**Recent Articles on Divisibility!

**Misc :**Recent Articles on Mathematical Algorithms!

Recent Articles on Number Theory

Above facts are contributed by

**Yash Kodesia**. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.Please write comments if you find anything incorrect, or you want to share more facts about Number Theory.

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