The Number Dictionary


About this project...

This project started in October 2008

The purpose is to provide an opportunity to show properties of numbers

Term
Definition
Examples
Prime number A whole number that has exactly two factors, 1 and itself. 2, 3, 5, 7, 11, 13, 17
Perfect number A number for which the sum of all the proper factors is equal to the number itself. 6, 28, 496, 8128
Triangular number A triangular number is the sum of the n natural numbers from 1 to n 1, 3, 6, 10, 15, 21
Fibonacci sequence The first number of the sequence is 0, the second number is 1, and each subsequent number is equal to the sum of the previous two numbers of the sequence itself, yielding the sequence 1, 2, 3, 5, 8, 13, 21
Goldbach conjecture The unestablished conjecture that every even number except the number 2 is the sum of two primes. 28 = 23 + 5 = 17 + 11
Perfect square An integer which is the square of some other integer, i.e. can be written in the form n2 for some integer n. 1, 4, 9, 16, 25, 36, 49
Number factorization The resolution of a number into factors such that when multiplied together they give the original number. 28 = 2 × 2 × 7
Sum of first primes The sum of the sequence 2, 3, 5, ... , pn where pn is the n(th) prime number 17 = 2 + 3 + 5 + 7
Sum of two squares The sum of two different perfect square numbers 25 = 42 + 32
Difference of two squares The difference of two different perfect square numbers 17 = 92 - 82

Recently added...
pentagonal numbers Figurate numbers with the form n*(3n-1)/2 1, 5, 12, 22, 35
hexagonal numbers Figurate numbers with the form n*(4n-2)/2 1, 6, 15, 28, 45
heptagonal numbers Figurate numbers with the form n*(5n-3)/2 1, 7, 18, 34, 55
octogonal numbers Figurate numbers with the form n*(3n-2) 1, 8, 21, 40, 65
Septimal numeral Numeral system with base 7.  
Numbers, inside of two twin primes Even numbers that are inside of two primes (twins). 11, 12, 13 (12)

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