constructible numbers
The smallest subfield of over such that is Euclidean is called the field of real constructible numbers. First, note that has the following properties:
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1.
;
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2.
If , then also , , and , the last of which is meaningful only when ;
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3.
If and , then .
The field can be extended in a natural manner to a subfield of that is not a subfield of . Let be a subset of that has the following properties:
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1.
;
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2.
If , then also , , and , the last of which is meaningful only when ;
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3.
If and where , then .
Then is the field of constructible numbers.
Note that . Moreover, .
An element of is called a constructible number. These numbers can be βconstructedβ by a process that will be described shortly.
Conversely, let us start with a subset of such that contains a non-zero complex number. Call any of the binary operations in condition 2 as well as the square root unary operation in condition 3 a ruler and compass operation. Call a complex number constructible from if it can be obtained from elements of by a finite sequence of ruler and compass operations. Note that . If is the set of numbers constructible from using only the binary ruler and compass operations (those in condition 2), then is a subfield of , and is the smallest field containing . Next, denote the set of all constructible numbers from . It is not hard to see that is also a subfield of , but an extension of . Furthermore, it is not hard to show that is Euclidean. The general process (algorithm) of elements in from elements in using finite sequences of ruler and compass operations is called a ruler and compass construction. These are so called because, given two points, one of which is 0, the other of which is a non-zero real number in , one can use a ruler and compass to construct these elements of .
If (or any rational number), we see that is the field of constructible numbers.
Note that the lengths of constructible line segments (http://planetmath.org/Constructible2) on the Euclidean plane are exactly the positive elements of . Note also that the set is in one-to-one correspondence with the set of constructible points (http://planetmath.org/Constructible2) on the Euclidean plane. These facts provide a between abstract algebra and compass and straightedge constructions.
Title | constructible numbers |
Canonical name | ConstructibleNumbers |
Date of creation | 2013-03-22 17:15:01 |
Last modified on | 2013-03-22 17:15:01 |
Owner | CWoo (3771) |
Last modified by | CWoo (3771) |
Numerical id | 17 |
Author | CWoo (3771) |
Entry type | Definition |
Classification | msc 12D15 |
Related topic | EuclideanField |
Related topic | CompassAndStraightedgeConstruction |
Related topic | TheoremOnConstructibleAngles |
Related topic | TheoremOnConstructibleNumbers |
Defines | ruler and compass operation |
Defines | compass and ruler operation |
Defines | compass and straightedge operation |
Defines | straightedge and compass operation |
Defines | constructible number |
Defines | constructible from |
Defines | constructible |
Defines | field of constructible numbers |
Defines | field of real constructible numbers |