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In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid for the diameter of a sphere. In more modern usage, the length of a diameter is also called the diameter. In this sense one speaks of the diameter rather than a diameter (which refers to the line itself), because all diameters of a circle or sphere have the same length, this being twice the radius r.

d = 2 r

r =

d 2

.

displaystyle d=2rquad Rightarrow quad r= frac d 2 .

For a convex shape in the plane, the diameter is defined to be the largest distance that can be formed between two opposite parallel lines tangent to its boundary, and the width is often defined to be the smallest such distance. Both quantities can be calculated efficiently using rotating calipers.[1] For a curve of constant width such as the Reuleaux triangle, the width and diameter are the same because all such pairs of parallel tangent lines have the same distance.

For an ellipse, the standard terminology is different. A diameter of an ellipse is any chord passing through the center of the ellipse.[2] For example, conjugate diameters have the property that a tangent line to the ellipse at the endpoint of one of them is parallel to the other one. The longest diameter is called the major axis. The word "diameter" is derived from Greek διάμετρος (diametros), "diameter of a circle", from διά (dia), "across, through" and μέτρον (metron), "measure".[3] It is often abbreviated DIA, dia, d, or ⌀.

Contents

1 Generalizations 2 Diameter
Diameter
symbol 3 See also 4 Notes

Generalizations[edit] The definitions given above are only valid for circles, spheres and convex shapes. However, they are special cases of a more general definition that is valid for any kind of n-dimensional convex or non-convex object, such as a hypercube or a set of scattered points. The diameter of a subset of a metric space is the least upper bound of the set of all distances between pairs of points in the subset. So, if A is the subset, the diameter is

sup d(x, y) x, y ∈ A .

If the distance function d is viewed here as having codomain R (the set of all real numbers), this implies that the diameter of the empty set (the case A = ∅) equals −∞ (negative infinity). Some authors prefer to treat the empty set as a special case, assigning it a diameter equal to 0,[4] which corresponds to taking the codomain of d to be the set of nonnegative reals. For any solid object or set of scattered points in n-dimensional Euclidean space, the diameter of the object or set is the same as the diameter of its convex hull. In medical parlance concerning a lesion or in geology concerning a rock, the diameter of an object is the supremum of the set of all distances between pairs of points in the object. In differential geometry, the diameter is an important global Riemannian invariant. In plane geometry, a diameter of a conic section is typically defined as any chord which passes through the conic's centre; such diameters are not necessarily of uniform length, except in the case of the circle, which has eccentricity e = 0. Diameter
Diameter
symbol[edit]

Sign ⌀ in a technical drawing

Sign ⌀ from an AutoCAD
AutoCAD
drawing

Not to be confused with the Scandinavian letter "Ø", the empty set symbol "∅" or the greek letter phi (Φ). The symbol or variable for diameter, ⌀, is similar in size and design to ø, the Latin small letter o with stroke. In Unicode
Unicode
it is defined as U+2300 ⌀ Diameter
Diameter
sign (HTML ⌀). On an Apple Macintosh, the diameter symbol can be entered via the character palette (this is opened by pressing ⌥ Opt⌘ CmdT in most applications), where it can be found in the Technical Symbols category. The character will sometimes not display correctly, however, since many fonts do not include it. In many situations the letter ø (the Latin small letter o with stroke) is an acceptable substitute, which in Unicode
Unicode
is U+00F8 ø (HTML ø · ø). It can be obtained in UNIX-like operating systems using a Compose key by pressing, in sequence, Composedi[citation needed] and on a Macintosh
Macintosh
by pressing ⌥ Opt O (the letter o, not the number 0). In Microsoft Word
Microsoft Word
the diameter symbol can be acquired by typing 2300 and then pressing Alt+X. In LaTeX
LaTeX
the diameter symbol can be obtained with the command diameter from the wasysym package. The diameter symbol ⌀ is distinct from the empty set symbol ∅, from an (italic) uppercase phi Φ, and from the Nordic vowel Ø.[5] See also slashed zero. In German, the diameter symbol (German Durchmesserzeichen) is also used as an average symbol (Durchschnittszeichen). See also[edit]

Look up diameter in Wiktionary, the free dictionary.

Angular diameter Caliper, micrometer, tools for measuring diameters Conjugate diameters Diameter
Diameter
(group theory), a concept in group theory Eratosthenes, who calculated the diameter of the Earth
Earth
around 240 BC. Graph or network diameter Hydraulic diameter Inside diameter Jung's theorem, an inequality relating the diameter to the radius of the smallest enclosing ball Sauter mean diameter Tangent lines to circles Ø
Ø
(other)

Notes[edit]

^ Toussaint, Godfried T. (1983). "Solving geometric problems with the rotating calipers". Proc. MELECON '83, Athens.  ^ Cut-the-Knot ^ Online Etymology Dictionary ^ Re: diameter of an empty set ^ Korpela, Jukka K. (2006), Unicode
Unicode
Explained, O'Reilly Media, Inc., pp. 23–24, ISBN 978-0-596-10121-3 .

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