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21st
21 (twenty-one) is the natural number following 20 and preceding 22. The current century is the 21st century AD, under the Gregorian calendar. In mathematics 21 is: * a composite number, its proper divisors being 1, 3 and 7, and a deficient number as the sum of these divisors is less than the number itself. * a Fibonacci number as it is the sum of the preceding terms in the sequence, 8 and 13. * the fifth Motzkin number. * a triangular number, because it is the sum of the first six natural numbers (1 + 2 + 3 + 4 + 5 + 6 = 21). * an octagonal number. * a Padovan number, preceded by the terms 9, 12, 16 (it is the sum of the first two of these) in the padovan sequence. * a Blum integer, since it is a semiprime with both its prime factors being Gaussian primes. * the sum of the divisors of the first 5 positive integers (i.e., 1 + (1 + 2) + (1 + 3) + (1 + 2 + 4) + (1 + 5)) * the smallest non-trivial example of a Fibonacci number whose digits are Fibonacci numbers and whose digit ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Natural Number
In mathematics, the natural numbers are those numbers used for counting (as in "there are ''six'' coins on the table") and ordering (as in "this is the ''third'' largest city in the country"). Numbers used for counting are called ''Cardinal number, cardinal numbers'', and numbers used for ordering are called ''Ordinal number, ordinal numbers''. Natural numbers are sometimes used as labels, known as ''nominal numbers'', having none of the properties of numbers in a mathematical sense (e.g. sports Number (sports), jersey numbers). Some definitions, including the standard ISO/IEC 80000, ISO 80000-2, begin the natural numbers with , corresponding to the non-negative integers , whereas others start with , corresponding to the positive integers Texts that exclude zero from the natural numbers sometimes refer to the natural numbers together with zero as the whole numbers, while in other writings, that term is used instead for the integers (including negative integers). The natural ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Gaussian Prime
In number theory, a Gaussian integer is a complex number whose real and imaginary parts are both integers. The Gaussian integers, with ordinary addition and multiplication of complex numbers, form an integral domain, usually written as \mathbf /math> or \Z Gaussian integers share many properties with integers: they form a Euclidean domain, and have thus a Euclidean division and a Euclidean algorithm; this implies unique factorization and many related properties. However, Gaussian integers do not have a total ordering that respects arithmetic. Gaussian integers are algebraic integers and form the simplest ring of quadratic integers. Gaussian integers are named after the German mathematician Carl Friedrich Gauss. Basic definitions The Gaussian integers are the set :\mathbf \, \qquad \text i^2 = -1. In other words, a Gaussian integer is a complex number such that its real and imaginary parts are both integers. Since the Gaussian integers are closed under addition and mul ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Voting Age
A voting age is a minimum age established by law that a person must attain before they become eligible to vote in a public election. The most common voting age is 18 years; however, voting ages as low as 16 and as high as 25 currently exist (see list below). Most countries have set a minimum voting age, often set in their constitution. In a number of countries voting is compulsory for those eligible to vote, while in most it is optional. When the right to vote was being established in democracies, the voting age was generally set at 21 or higher. In the 1970s many countries reduced the voting age to 18. The debate is ongoing in a number of countries on proposals to reduce the voting age to or below 18. In Brazil, for example, the minimum age lowered from 18 to 16 years old in the 1988 constitution. History In 1890, Law No. 5, 1890, of the South African Republic, commonly known as Transvaal, set a voting age there of 18 years. The effort was, like later legislation expandin ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Legal Drinking Age
The legal drinking age is the minimum age at which a person can legally consume alcoholic beverages. The minimum age alcohol can be legally consumed can be different from the age when it can be purchased in some countries. These laws vary between countries and many laws have exemptions or special circumstances. Most laws apply only to drinking alcohol in public places with alcohol consumption in the home being mostly unregulated (an exception being the UK, which has a minimum legal age of five for supervised consumption in private places). Some countries also have different age limits for different types of alcohol drinks. The majority of countries have a minimum legal drinking age of 18. The most commonly known reason for the law behind the legal drinking age is the effect on the brain in adolescents. Since the brain is still maturing, alcohol can have a negative effect on the memory and long-term thinking. Alongside that, it can cause liver failure, and create a hormone imb ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Smoking Age
smoking age is the minimum legal age required to purchase or use tobacco products. Most countries have laws that forbid sale of tobacco products to persons younger than certain ages, usually the age of majority. This article does not discuss laws that regulate electronic cigarettes. Laws by region Africa Americas Asia Europe Oceania Historical regulations File:Verkaufsalter für Tabakwaren in Europa (im Jahr 1995).svg, Minimum age to purchase tobacco in the European Union as of 1995: File:Verkaufsalter für Tabakwaren in Nordamerika (im Jahr 1989).svg, Minimum age to purchase tobacco in the USA as of 1989: File:Smoking Age - USA before December 2019.svg, Minimum age to purchase tobacco in the US prior to December 20, 2019: Cannabis age Since 2012, various jurisdictions throughout the world have legalized cannabis for recreational use. In Mexico, Uruguay and cannabis-legal jurisdictions in the United States, the legal age to possess ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Coming Of Age
Coming of age is a young person's transition from being a child to being an adult. The specific age at which this transition takes place varies between societies, as does the nature of the change. It can be a simple legal convention or can be part of a ritual or spiritual event, as practiced by many societies. In the past, and in some societies today, such a change is associated with the age of sexual maturity (puberty), especially menarche and spermarche. In others, it is associated with an age of religious responsibility. Particularly in western societies, modern legal conventions which stipulate points in around the end of adolescence and the beginning of early adulthood (most commonly 18, with the range being 16-21) when adolescents are generally no longer considered minors and are granted the full rights and responsibilities of an adult) are the focus of the transition. In either case, many cultures retain ceremonies to confirm the coming of age, and coming-of-age storie ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Age Of Majority
The age of majority is the threshold of legal adulthood as recognized or declared in law. It is the moment when minors cease to be considered such and assume legal control over their persons, actions, and decisions, thus terminating the control and legal responsibilities of their parents or guardian over them. Most countries set the age of majority at 18, but some jurisdictions have a higher age and others lower. The word ''majority'' here refers to having greater years and being of full age as opposed to ''minority'', the state of being a minor. The law in a given jurisdiction may not actually use the term "age of majority". The term typically refers to a collection of laws bestowing the status of adulthood. Those under the age of majority are referred to as minors and may be legally denied certain privileges or rights (e.g. the right to vote, buy alcohol, marry, sign a binding contract). Age of majority should not be confused with the age of maturity, age of sexual consent, ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Solstice
A solstice is an event that occurs when the Sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around June 21 and December 21. In many countries, the seasons of the year are determined by the solstices and the equinoxes. The term ''solstice'' can also be used in a broader sense, as the day when this occurs. The day of a solstice in either hemisphere has either the most sunlight of the year ( summer solstice) or the least sunlight of the year (winter solstice) for any place other than the Equator. Alternative terms, with no ambiguity as to which hemisphere is the context, are " June solstice" and " December solstice", referring to the months in which they take place every year. The word ''solstice'' is derived from the Latin ''sol'' ("sun") and ''sistere'' ("to stand still"), because at the solstices, the Sun's declination appears to "stand still"; that is, the seasonal move ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Scandium
Scandium is a chemical element with the symbol Sc and atomic number 21. It is a silvery-white metallic d-block element. Historically, it has been classified as a rare-earth element, together with yttrium and the Lanthanides. It was discovered in 1879 by spectral analysis of the minerals euxenite and gadolinite from Scandinavia. Scandium is present in most of the deposits of rare-earth and uranium compounds, but it is extracted from these ores in only a few mines worldwide. Because of the low availability and difficulties in the preparation of metallic scandium, which was first done in 1937, applications for scandium were not developed until the 1970s, when the positive effects of scandium on aluminium alloys were discovered, and its use in such alloys remains its only major application. The global trade of scandium oxide is 15–20 tonnes per year. The properties of scandium compounds are intermediate between those of aluminium and yttrium. A diagonal relationship exists betwee ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Atomic Number
The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of every atom of that element. The atomic number can be used to uniquely identify ordinary chemical elements. In an ordinary uncharged atom, the atomic number is also equal to the number of electrons. For an ordinary atom, the sum of the atomic number ''Z'' and the neutron number ''N'' gives the atom's atomic mass number ''A''. Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes) and the mass defect of the nucleon binding is always small compared to the nucleon mass, the atomic mass of any atom, when expressed in unified atomic mass units (making a quantity called the "relative isotopic mass"), is within 1% of the whole number ''A''. Atoms with the same atomic number but dif ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Squaring The Square
Squaring the square is the problem of tiling an integral square using only other integral squares. (An integral square is a square whose sides have integer length.) The name was coined in a humorous analogy with squaring the circle. Squaring the square is an easy task unless additional conditions are set. The most studied restriction is that the squaring be perfect, meaning the sizes of the smaller squares are all different. A related problem is squaring the plane, which can be done even with the restriction that each natural number occurs exactly once as a size of a square in the tiling. The order of a squared square is its number of constituent squares. Perfect squared squares A "perfect" squared square is a square such that each of the smaller squares has a different size. It is first recorded as being studied by R. L. Brooks, C. A. B. Smith, A. H. Stone and W. T. Tutte at Cambridge University between 1936 and 1938. They transformed the square tiling into an equivalent el ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Quaternary Numeral System
A quaternary numeral system is base-. It uses the digits 0, 1, 2 and 3 to represent any real number. Conversion from binary is straightforward. Four is the largest number within the subitizing range and one of two numbers that is both a square and a highly composite number (the other being 36), making quaternary a convenient choice for a base at this scale. Despite being twice as large, its radix economy is equal to that of binary. However, it fares no better in the localization of prime numbers (the smallest better base being the primorial base six, senary). Quaternary shares with all fixed- radix numeral systems many properties, such as the ability to represent any real number with a canonical representation (almost unique) and the characteristics of the representations of rational numbers and irrational numbers. See decimal and binary for a discussion of these properties. Relation to other positional number systems Relation to binary and hexadecimal As with the o ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
