20

The number 20 is a natural number that follows 19 and precedes 21. It is a prime number whose prime factors are 2 and 5, and its factors include 1, 2, 4, 5, 10, and 20. As the sum of its factors (22) is greater than itself, 20 is considered an abundant number. It can be used as a base for number systems, such as the Roman and Maya numerals, where it remains a powerful symbol. In mathematics, 20 is a pronic number, the third composite number to be a product of a squared prime and a prime, and the first number to have an abundance of 2. Its decimal representation is 20, the smallest non-trivial neon number equal to the sum of its digits when raised to the thirteenth power. The 20th prime number, it is also the 20th indexed prime number in the sequence of self-convolved Fibonacci numbers, where the largest superingular prime factor that divides the order of the friendly giant is 71. In science, chemistry, and biology, 20 holds various significance, notably as the atomic number of calcium, a component of proteins, a measure of visual acuity, and a standard in sports and games. In culture, 20 serves as an index for visual acuity and milestones in life, historically marking the age of majority in Japan. The 20th month of the year, March, is also named after the number. It is a uniform tiling of the plane by regular hexagons and has many other meanings and uses in various fields, from board games to abstract algebra.