## Faqts : Science : Mathematics : Number : Complex

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### Entry

##### Math: Transform: Fourier: Can you tell more about f(x) . e^( x . i )? [fast Fourier transform]

Aug 27th, 2009 00:37
techi gity, Knud van Eeden,

```----------------------------------------------------------------------
--- Knud van Eeden --- 16 April 2005 - 00:41 am ----------------------
Math: Transform: Fourier: Can you tell more about f(x) . e^( x . i )?
[fast Fourier transform]
---
1. The definition of a complex number (in a rectangular coordinate
system) is:
z = (real number) + (imaginary number)
or thus
(horizontal component) + (vertical component) . i
is a representation of a vector
x . i
2. It can be shown that e      is just a special case of such a vector
(that is it is a unit vector (thus with length 1), rotated over an
angle x, in a unit circle.
x . i
3. So e      is a unit vector (with length 1), rotated over an angle
x, in a unit circle.
x . i
4. 1 . e      is a scaled unit vector (with length 1), rotated over
an angle x, in a unit circle.
x . i
5. 8 . e      is a scaled vector (with length equal to 8), rotated
over an angle x, in a circle.
x . i
6. constant . e      is a scaled vector (with length equal to this
constant), rotated over an angle x, in a circle.
x . i
7. y . e      is a scaled vector (with length equal to this y),
rotated over an angle x, in a circle.
x . i
8. f(x) . e      is a scaled vector (with length equal to this f
(x) ), rotated over an angle x, in a circle.
1. x . i
9. f(x) . e          is a scaled vector (with length equal to this f
(x) ), rotated over an angle 1 . x, in a circle.
8 . x . i
10. f(x) . e          is a scaled vector (with length equal to this f
(x) ), rotated over an angle 8 . x, in a circle.
8 . x . i + 1
11. f(x) . e               is a scaled vector (with length equal to
this f(x) ), rotated over an angle 8 . x and starting at an angle 1,
in a circle.
8 . x . i + 4
12. f(x) . e               is a scaled vector (with length equal to
this f(x) ), rotated over an angle 8 . x and starting at an angle 4,
in a circle.
8 . x . i + constant1
13. f(x) . e                      is a scaled vector (with length
equal to this f(x) ), rotated over an angle 8 . x and starting at an
angle constant1, in a circle.
constant2 . x . i + constant1
14. f(x) . e                              is a scaled vector (with
length equal to this f(x) ), rotated over an angle constant2 . x and
starting at an angle constant1, in a circle.
scale.(independent variable).i+translation
15. (dependent variable).e
is a scaled vector (with length equal to this dependent variable),
rotated over an angle ('scalefactor' times 'independent variable')
plus that translationconstant, in a circle.
---
---
Summarizing:
---
Steps: Overview:
1. -Choose the value of your independent variable
(e.g. 1, 2, 3, ...)
2. -Calculate the value of the corresponding dependent
variable (e.g if y = x^2, you get 1^2, 2^2, 3^2, ...
or thus 1, 4, 9, ...)
3. -Choose a scalefactor for the rotation
4. -Choose a translationconstant for the rotation
5. -Then you rotate that angle of that vector first over an angle
equal to that scalefactor times the given independent variable.
6. -You add the translation constant to this result
/
/
/ angle equal to this scalefactor times independent variable
plus a translationconstant, thus an angle equal to e.g.  constant2 . 1
+ constant1, constant2 . 2 + constant1, constant 2 . 3 + constant1, ...
-----
7. -Then you finally scale the length of the vector to be equal to
that calculated value of the dependent variable
/
/
/
/
/ length equal to this independent variable, thus an length
equal to e.g.  1, 4, 9, ...
-----
8. -If you need to create a sum of this vectors, as e.g. in a Fourier
Transform, you just vector add this vectors
1. -Add the horizontal component of each vector to a sum
2. -Add the vertical component of each vector to a sum
9. -This will give thus a final resulting vector representing this
sums
---
---
Note:
The whole idea is thus similar to using polar coordinates,
where you have an angle and a scaled radius length.
---
---
Note:
You see thus clearly that this is a special case of a transformation
(as a transformation is per definition just some combination of
operations like e.g. scaling, translation, rotation, ...)
---
---
---
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