"Seventeen" redirects here. For other uses, see 17 (disambiguation). 17 (seventeen) is the natural number following 16 and preceding 18. It is prime. In spoken English, the numbers 17 and 70 are sometimes confused because they sound similar. When carefully enunciated, they differ in which syllable is stressed: 17 /sɛvɨnˈtiːn/ vs 70 /ˈsɛvɨnti/. However, in dates such as 1789 or when contrasting numbers in the teens, such as 16, 17, 18, the stress shifts to the first syllable: 17 /ˈsɛvɨntiːn/. The number 17 has wide significance in pure mathematics, as well as in applied sciences, law, music, religion, sports, and other cultural phenomena. In mathematics: Seventeen is the 7th prime number. The next prime is nineteen, with which it forms a twin prime. 17 is the sum of the first four primes. 17 is the sixth Mersenne prime exponent, yielding 131071. 17 is an Eisenstein prime with no imaginary part and real part of the form 3n − 1. 17 is the third Fermat prime, as it is of the form specifically with n = 2, and it is also a Proth prime. Since 17 is a Fermat prime, regular heptadecagons can be constructed with compass and unmarked ruler. This was proven by Carl Friedrich Gauss. Another consequence of 17 being a Fermat prime is that it is not a Higgs prime for squares or cubes; in fact, it is the smallest prime not to be a Higgs prime for squares, and the smallest not to be a Higgs prime for cubes. 17 is the only positive Genocchi number that is prime, the only negative one being −3. It is also the third Stern prime. 17 is the thirteenth term of the Euclid-Mullin sequence. Seventeen is the aliquot sum of the semiprime 39, and is the aliquot sum of the semiprime 55, and is the base of the 17-aliquot tree. There are exactly 17 two-dimensional space (plane symmetry) groups. These are sometimes called wallpaper groups, as they represent the seventeen possible symmetry types that can be used for wallpaper. Like 41, the number 17 is a prime that yields primes in the polynomial n + n + p, for all positive n < p − 1. In the Irregularity of distributions problem, consider a sequence of real numbers between 0 and 1 such that the first two lie in different halves of this interval, the first three in different thirds, and so forth. The maximum possible length of such a sequence is 17 (Berlekamp & Graham, 1970, example 63). Either 16 or 18 unit squares can be formed into rectangles with perimeter equal to the area; and there are no other natural numbers with this property. The Platonists regarded this as a sign of their peculiar propriety; and Plutarch notes it when writing that the Pythagoreans "utterly abominate" 17, which "bars them off from each other and disjoins them". 17 is the tenth Perrin number, preceded in the sequence by 7, 10, 12. In base 9, the smallest prime with a composite sum of digits is 17. 17 is the least random number, according to the Hackers' Jargon File. It is a repunit prime in hexadecimal (11). 17 is the minimum possible number of givens for a sudoku puzzle with a unique solution. This was long conjectured, and was proved in 2012. There are 17 orthogonal curvilinear coordinate systems (to within a conformal symmetry) in which the 3-variable Laplace equation can be solved using the separation of variables technique. 17 is the first number that can be written as the sum of a positive cube and a positive square in two different ways; that is, the smallest n such that x + y = n has two different solutions for x and y positive integers. The next such number is 65. 17 is the minimum number of vertices on a graph such that, if the edges are coloured with 3 different colours, there is bound to be a monochromatic triangle. (See Ramsey's Theorem.) 17 is a full reptend prime in base 10, because its repeating decimal is 16 digits long. In science: The atomic number of chlorine., The Brodmann area defining the primary visual processing area of mammalian brains., Group 17 of the periodic table is called the halogens., The number of elementary particles in the Standard Model of physics., Age 17: In the UK, the minimum driving age for a car or van., In the US and Canada, it is the age at which one may purchase, rent, or reserve M-rated video games without parental consent., Also, in some US states, and some jurisdictions around the world, 17 is the age of sexual consent., In most US states, Canada and in the UK, the age at which one may donate blood (without parental consent)., In many countries and regions, the age at which one may obtain a driver's license., In the US, the age at which one may watch, rent, or purchase R-rated movies without parental consent., In the US, the age at which one can enlist in the armed forces with parental permission., At this age one can apply for a Private Pilot Licence (however the applicant can start training at 16)., In culture: Music: Bands: 17 Hippies, a German band, Sytten (17 in Norwegian), a Norwegian-American rapper

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