The number 17 is a natural number following 16 and preceding 18. It is a prime number and has many interesting mathematical properties. For example, 17 is a Fermat prime, one of the lucky numbers of Euler, and a Leyland number. It is the smallest number for which a regular heptadecagon (a 17-sided polygon) can be constructed with a compass and an unmarked ruler, a fact famously proven by Carl Friedrich Gauss. In mathematics and geometry, 17 appears in several notable contexts such as 17 crystallographic space groups in two dimensions, 17 distinct stellations of an icosahedron, and 17 being the minimum number of givens for a Sudoku puzzle with a unique solution. Outside of mathematics, the number 17 has significance in other fields:
- It is the atomic number of chlorine.
- It is the number of syllables in a traditional Japanese haiku (5-7-5).
- It has cultural and symbolic meanings, such as being considered the "least random number" in psychology studies where people pick a random number between 1 and 20.
Additionally, 17 is notable in various domains such as music, art, and literature, often appearing as a symbolic or reference number.