algae \'al-je\ Members of a group of mostly aquatic, photosynthetic organisms (see photosynthesis) that defy precise definition. They range in size from the microscopic flagellate Micromonas to giant kelp that reach 200 ft (60 m) in length. Algae provide much of Earth’s oxygen, serve as the food base for almost all aquatic life, and provide foods and industrial products, including petroleum products. Their photosynthetic pigments are more varied than those of plants, and their cells have features not found among plants and animals. The classification of algae is changing rapidly because new taxonomic information is being discovered. Algae were formerly classified into three major groups—the red, brown, and green seaweeds—based on the pigment molecules in their chloroplasts. Many more than three groups are now recognized, each sharing a com¬ mon set of pigment types. Algae are not closely related to each other in an evolutionary sense. Specific groups can be distinguished from proto¬ zoans and fungi (see fungus) only by the presence of chloroplasts and by their ability to carry out photosynthesis; these specific groups thus have a closer evolutionary relationship with the protozoa or fungi than with other algae. Algae are common on “slimy” rocks in streams (see diatoms) and as green sheens on pools and ponds. Use of algae is perhaps as old as humankind; many species are eaten by coastal societies.
Algardi, Alessandro (b. 1595, Bologna, Papal States—d. June 10, 1654, Rome) Italian sculptor. He trained in Bologna under the Car¬ racci family and in 1625 moved to Rome, where he designed the stucco decorations in San Silvestro al Quiri- nale. He later became the most out¬ standing Baroque sculptor in Rome after Gian Lorenzo Bernini. He was a prolific sculptor of portrait busts, and his colossal marble relief of the Meeting of Attila and Pope Leo (1646-53) in St. Peter’s Basilica influenced the development and popularity of illusionistic reliefs. His work as a restorer of antique statu¬ ary brought him some notoriety.
algebra, linear See linear algebra
algebra and algebraic struc¬ tures Generalized version of arith¬ metic that uses variables to stand for unspecified numbers. Its purpose is tO Solve ALGEBRAIC EQUATIONS Or SYSTEMS of equations. Examples of such solu¬ tions are the quadratic formula (for solving a quadratic equation) and Gauss-Jordan elimination (for solv¬ ing a SYSTEM OF EQUATIONS in MATRIX form). In higher mathematics, an “algebra” is a structure consisting of a
class of objects and a set of rules (analogous to addition and multiplica¬ tion) for combining them. Basic and higher algebraic structures share two essential characteristics: (1) calculations involve a finite number of steps and (2) calculations involve abstract symbols (usually letters) represent¬ ing more general objects (usually numbers). Higher algebra (also known as modem or abstract algebra) includes all of elementary algebra, as well as group theory, theory of rings, field theory, manifolds, and vector spaces.
algebraic equation Mathematical statement of equality between algebraic expressions. An expression is algebraic if it involves a finite combination of numbers and variables and algebraic operations (addition, subtraction, multiplication, division, raising to a power, and extracting a root). Two important types of such equations are linear equations, in the form y = ax + b, and quadratic equations, in the form y = ax 2 + bx + c. A solution is a numerical value that makes the equation a true statement when substituted for a variable. In some cases it may be found using a formula; in others the equation may be rewritten in simpler form. Alge¬ braic equations are particularly useful for modeling real-life phenomena.
algebraic geometry Study of geometric objects expressed as equa¬ tions and represented by graphs in a given coordinate system. In contrast to Euclidean geometry, algebraic geometry represents geometric objects using algebraic equations (e.g., a circle of radius r is defined by x 2 + y 2 = r 2 ). Objects so defined can then be analyzed for symmetries, intercepts, and other properties without having to refer to a graph.
algebraic topology Field of mathematics that uses algebraic struc¬ tures to study transformations of geometric objects. It uses functions (often called maps in this context) to represent continuous transforma¬ tions (see topology). Taken together, a set of maps and objects may form an algebraic group, which can be analyzed by group-theory methods. A well-known topic in algebraic topology is the four-colour map problem.
Algeciras \,al-j9-'sir-9s\ Conference (Jan. 16-April 7, 1906) Confer¬ ence held at Algeciras, Spain, that resolved the first of the Moroccan crises. In 1905 William II objected to France’s efforts to exert influence in Morocco, prompting a conference of the European powers and the U.S. Superficially, the Act of Algeciras (1906) seemed to limit French penetration, but the con¬ ference’s real significance was the diplomatic support that Britain and the U.S. gave France, foreshadowing their roles in World War I.
Alger Val-jor\, Horatio, Jr. (b. Jan. 13, 1832, Chelsea, Mass., U.S.—d. July 18, 1899, Natick, Mass.) U.S. writer. The son of a Unitarian minis¬ ter, Alger graduated from Harvard with honours and then earned a degree from its divinity school. Forced to leave his pulpit after two years because of allegations of improper activities with youths, he took up writing. Beginning with Ragged Dick (1868), he wrote more than 100 books that were almost alike in preaching that through honesty, cheerful persever¬ ance, and hard work a poor but virtuous lad would have his just reward (though it was almost always precipitated by good luck). His books sold more than 20 million copies, despite consistently weak plots and dialogue, and Alger was one of the most popular and socially influential writers of the late 19th century.
Algeria officially People's Democratic Republic of Algeria
Country, North Africa. Area: 919,595 sq mi (2,381,741 sq km). Popula¬ tion (2005 est.): 32,854,000. Capitaclass="underline" Algiers. Most of the population is ethnically and linguistically Arab, with a large Berber (Amazigh) minor¬ ity. Languages: Arabic, Berber (both official), French. Religion: Islam (official; predominantly Sunni). Currency: Algerian dinar. Algeria has the second largest land area (after The Sudan) on the continent. The coast¬ line has numerous bays, and the country’s rivers are small and generally seasonal. Northern Algeria is mountainous and is crossed from east to west by the Atlas Mountains; its highest point, elevation 7,638 ft (2,328 m), is Mount Chelia. In central and southern Algeria is much of the north¬ ern Sahara. Algeria has a developing economy based primarily on the production and export of petroleum and natural gas. After achieving inde¬ pendence, the country nationalized much of its economy but since the 1980s has privatized parts of the economy. Algeria is a republic with two legislative bodies; its chief of state is the president, and its head of gov¬ ernment is the prime minister. Phoenician traders settled there early in the 1st millennium bc; several centuries later the Romans invaded, and by ad 40 they had control of the Mediterranean coast. The fall of Rome in the 5th century led to an invasion by the Vandals and later to a reoccupation by the Byzantine (Eastern Roman) Empire. The Islamic invasion began in the 7th century; by 711 all of northern Africa was under the control of the caliphs of the Umayyad dynasty. Several Islamic Berber empires fol-
"Meeting of Attila and Pope Leo," colossal marble relief by Alessandro Algardi, 1646-53; in St. Peter's, Rome
AUNARI—ANDERSON FROM ART RESOURCE/EB INC.
© 2006 Encyclopaedia Britannica, Inc.
Algerian Reformist ‘Ulama’ ► 'AIT I 47
lowed, most prominently the Almoravid (c. 1054-1130), which extended its domain to Spain, and the Almohad (c. 1130-1269). The Barbary Coast pirates menaced Medi¬ terranean trade for centuries; their raids served as a pretext for France to enter Algeria in 1830. By 1847 France had established military control over most of the region and by the late 19th century had instituted civil rule. Popular protest against French rule resulted in the bloody Algerian War (1954—61); independence was achieved following a referendum in 1962. Beginning in the early 1990s, Islamic fundamentalist opposition to secular rule led to an outbreak in civil violence between the army and various Islamic extremist groups.