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Mathematics is the study of numbers, quantity, space, pattern, structure, and change. Mathematics is used throughout the world as an essential tool in many fields, including natural science, engineering, medicine, and the social sciences. It is used for calculation and considered as the most important subject. Applied mathematics, the branch of mathematics concerned with application of mathematical knowledge to other fields, inspires and makes use of new mathematical discoveries and sometimes leads to the development of entirely new mathematical disciplines, such as statistics and game theory. Mathematicians also engage in pure mathematics, or mathematics for its own sake, without having any application in mind. There is no clear line separating pure and applied mathematics, and practical applications for what began as pure mathematics are often discovered. (Full article...)

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All of the trigonometric functions of an angle θ can be constructed geometrically in terms of a unit circle centered at O.
Image credit: User:Tttrung

The trigonometric functions are functions of an angle; they are most important when studying triangles and modeling periodic phenomena, among many other applications. They are commonly defined as ratios of two sides of a right triangle containing the angle, and can equivalently be defined as the lengths of various line segments from a unit circle. More modern definitions express them as infinite series or as solutions of certain differential equations, allowing their extension to positive and negative values and even to complex numbers.

The study of trigonometric functions dates back to Babylonian times, and a considerable amount of fundamental work was done by ancient Greek, Indian and Arab mathematicians. (Full article...)

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Ordinal numbers

Credit: Pop-up casket & Fool

This spiral diagram represents all ordinal numbers less than

ω ω

. The first (outermost) turn of the spiral represents the finite ordinal numbers, which are the regular counting numbers starting with zero. As the spiral completes its first turn (at the top of the diagram), the ordinal numbers approach infinity, or more precisely

ω

, the first transfinite ordinal number (identified with the set of all counting numbers, a "countably infinite" set, the cardinality of which corresponds to the first transfinite cardinal number, called

ℵ 0

). The ordinal numbers continue from this point in the second turn of the spiral with

ω + 1

,

ω + 2

, and so forth. (A special ordinal arithmetic is defined to give meaning to these expressions, since the

+

symbol here does not represent the addition of two real numbers.) Halfway through the second turn of the spiral (at the bottom) the numbers approach

ω + ω

, or

ω \cdot 2

. The ordinal numbers continue with

ω \cdot 2 + 1

through

ω \cdot 2 + ω = ω \cdot 3

(three-quarters of the way through the second turn, or at the "9 o'clock" position), then through

ω \cdot 4

, and so forth, up to

ω \cdot ω = ω 2

at the top. (As with addition, the multiplication and exponentiation operations have definitions that work with transfinite numbers.) The ordinals continue in the third turn of the spiral with

ω 2 + 1

through

ω 2 + ω

, then through

ω 2 + ω 2 = ω 2 \cdot 2

, up to

ω 2 \cdot ω = ω 3

at the top of the third turn. Continuing in this way, the ordinals increase by one power of

ω

for each turn of the spiral, approaching

ω ω

in the middle of the diagram, as the spiral makes a countably infinite number of turns. This process can actually continue (not shown in this diagram) through

\omega ^{\omega ^{\omega }}

and

\omega ^{\omega ^{\omega ^{\omega }}}

, and so on, approaching the first epsilon number,

ε 0

. Each of these ordinals is still countable, and therefore equal in cardinality to

ω

. After uncountably many of these transfinite ordinals, the first uncountable ordinal is reached, corresponding to only the second infinite cardinal,

\aleph _{1}

. The identification of this larger cardinality with the cardinality of the set of real numbers can neither be proved nor disproved within the standard version of axiomatic set theory called Zermelo�CFraenkel set theory, whether or not one also assumes the axiom of choice.

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...that in senary, all prime numbers other than 2 and 3 end in 1 or a 5?
... if the integer n is prime, then the nth Perrin number is divisible by n?
...that it is impossible to trisect a general angle using only a ruler and a compass?
...that in a group of 23 people, there is a more than 50% chance that two people share a birthday?
...that the 1966 publication disproving Euler's sum of powers conjecture, proposed nearly 200 years earlier, consisted of only two sentences?
...the hyperbolic trigonometric functions of the natural logarithm can be represented by rational algebraic fractions?
... that economists blame market failures on non-convexity?

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Index of mathematics articles

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