441. Algebraic Geometry - JulTob/Mathematics GitHub Wiki

Algebraic Geometry

%%{init: {"quadrantChart": {"xAxisPosition": "bottom", "yAxisPosition": "left", "xRange": [-10, 10], "yRange": [-10, 10]}}} "titlePadding": 20}}}%%

quadrantChart
    title Cartesian Plane
    x-axis x
    y-axis y

    P1: [0.75, 0.75]
    P2: [0.45, 0.23]
    Origin: [0, 0] radius: 5, color: #444444, stroke-color: #ffffff, stroke-width: 3px
    x : [0.1, 0] radius: 5, color: #666666, stroke-color: #ff0000, stroke-width: 3px
    y : [0, 0.1] radius: 5, color: #666666, stroke-color: #00ff00, stroke-width: 3px


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The great Descartes made a connection between algebra and geometry. He developed the Cartesian Plane.

In it, we can define algebraically different geometric points.

Here are two simple points:

  • $P_1 = (x_1, y_1)$
  • $P_2 = (x_2, y_2)$

Parametric Equation of a Line: Point-Direction Form

We can then define the line that goes through both points.

$$\color{cyan} L = P_1 + mยทx$$

Where:

  • $m = \frac{y_2-y_1}{x_2-x_1}$ is the slope of the line.
  • $m = \tan{๐œƒ}$, where $๐œƒ$ is the angle with $๐‘ฅฬ‚$.

Generalized Line Equation

The equation of the line through the point $P = (x_1, y_1, z_1)$ and pointing in the direction of the vector $\vec{d} = a\hat{i} + b\hat{j} + c\hat{k}$ is:

$$R = P + t\vec{d}$$

Where:

  • $R = (x, y, z)$ is the general point on the line.
  • $t$ is a parameter that takes on all real values.

In coordinate form, the equations are:

$$\color{cyan} x = x_1 + at, \quad y = y_1 + bt, \quad z = z_1 + ct.$$

The parametric equations of the line l through the points $P = (x_1, y_1, z_1)$ and $Q = (x_2, y_2, z_2)$ are

$$\color{cyan} x = x_1 + (x_2 - x_1)t, \quad y = y_1 + (y_2 - y_1)t, \quad z = z_1 + (z_2 - z_1)t.$$
%%{init: {"quadrantChart": {"pointRadius": 3, "pointTextPadding": 13, "titlePadding": 20}}}%%

quadrantChart
    title Line
    x-axis x
    y-axis y
    P1: [0.75, 0.6] radius: 5, color: #7560ff
    P2: [0.45, 0.23] radius: 5, color: #4523ff
    D: [0.3, 0.37] radius: 5, color: #a57c00

    l1: [0.48, 0.267] color: #00FFFF

    l2: [0.51, 0.304] color: #00FFFF

    l3: [0.54, 0.34099999999999997] color: #00FFFF

    l4: [0.5700000000000001, 0.378] color: #00FFFF

    l5: [0.6000000000000001, 0.41500000000000004] color: #00FFFF

    l6: [0.63, 0.45199999999999996] color: #00FFFF

    l7: [0.66, 0.489] color: #00FFFF

    l8: [0.69, 0.526] color: #00FFFF

    l9: [0.72, 0.5630000000000001] color: #00FFFF

Loading
# for points 
(x1, y1) = (0.75, 0.6) # P1
(x2, y2) = (0.45, 0.23) # P2
L = 10 # Line Points
dL = 1/L # line segment size
(dx, dy) = (x1-x2, y1-y2) # Vector distance

for t in range(0,L+1):
    print(f"    l{t}: [{x2 + (dx)*t*dL}, {y2+(dy)*t*dL}] color: #00FFFF\n")

Displacement and Velocity

If an object has a (constant) velocity vector $\vec{v}$, then in $t$ units of time the resulting displacement vector of the object is

$$\color{#1216FF} \vec{d} = tยท\vec{v}$$
Example

Consider a seagull that flies in calm air with velocity vector $\vec{v}$. If a wind comes up with velocity $\vec{w}$ and the seagull continues flying the same way, its actual velocity will be $\vec{v}$ + $\vec{w}$. One can see the direction of the vector $\vec{v}$ because it points along the axis of the seagull. By comparing the direction of actual motion with the direction of $\vec{v}$, you can get an idea of the wind direction

Vectors

$$๐ฏโƒ—$$

Ordered List of elements that are part of a Vector Space.

$$โŸฎ๐‘ฃโ‚,๐‘ฃโ‚‚,๐‘ฃโ‚ƒ .,. ๐‘ฃโ‚™โŸฏ โˆˆ ๐•Œโฟ$$

Length , magnitude, norm

$$|๐ฏโƒ—|โ‰” \sqrt{โˆ‘ ๐‘ฃโ‚™ยฒ }$$

Vector Space

Defined from an origin

$$๐ŸŽโƒ—$$

Unit vector

$๐ฏฬ‚$ :

  • $|๐ฏฬ‚|=1$
  • $๐ฏฬ‚โˆฅ๐ฏโƒ—$

Scaling

$๐›‚๐ฏโƒ—$ :

  • $| ๐›‚๐ฏโƒ— |= ๐›‚| ๐ฏโƒ— |$
  • $๐›‚๐ฏโƒ— โˆฅ๐ฏโƒ—$

Adding

๐ฎโƒ—+๐ฏโƒ— = ๐ฐโƒ—

  • ๐ฐแตข = ๐ฎแตข+๐ฏแตข

๐ฎโƒ—,๐ฏโƒ—,๐ฐโƒ—โˆˆ ๐•Œโฟ

Vector Algebra

๐ฎโƒ—+๐ฏโƒ—= ๐ฏโƒ—+๐ฎโƒ—

๐ฎโƒ—+๐ฏโƒ—+๐ฐโƒ— = ๐ฎโƒ—+ (๐ฏโƒ—+๐ฐโƒ—) = (๐ฎโƒ—+๐ฏโƒ—)+๐ฐโƒ—

๐ฎโƒ—+ ๐ŸŽโƒ— = ๐ฎโƒ—

๐ฎโƒ—- ๐ฎโƒ— = ๐ŸŽโƒ—

|๐ฏโƒ—โˆฃโ‰ฅ0

|๐ฏโƒ—|=0โŸท๐ฏโƒ—= ๐ŸŽโƒ—

|๐ฏโƒ—โˆฃ= |-๐ฏโƒ—โˆฃ

|๐ฎโƒ—+๐ฏโƒ—|โ‰ค |๐ฎโƒ—โˆฃ+|๐ฏโƒ—โˆฃ

๐›ผ๐ฏโƒ— = ๐ฏโƒ—๐›ผ

๐›ผ(๐›ฝ๐ฏโƒ—)= (๐›ผ๐›ฝ)๐ฏโƒ—

๐›ผ(๐ฎโƒ—+๐ฏโƒ—)= ๐›ผ๐ฎโƒ—+๐›ผ๐ฏโƒ—

(๐›ผ+๐›ฝ)๐ฏโƒ— = ๐›ผ๐ฏโƒ—+ ๐›ฝ๐ฏโƒ—

1๐ฏโƒ—= ๐ฏโƒ—

0๐ฏโƒ—= ๐ŸŽโƒ— ๐›ผ๐ŸŽโƒ—= ๐ŸŽโƒ—

๐ฏโƒ—โˆ• ๐›ผ = ๐ฏโƒ—(1 โˆ• ๐›ผ)

๐ฏฬ‚=๐ฏโƒ—โˆ• |๐ฏโƒ—|

Space

๐•Œโฟ

  • ๐ŸŽโƒ—โˆˆ ๐•Œโฟ
  • ๐ฏโƒ— โˆˆ ๐•ŒโฟโŸถ ๐›ผ๐ฏโƒ— โˆˆ ๐•Œโฟ
  • ๐ฎโƒ—,๐ฏโƒ—โˆˆ ๐•Œโฟ โŸถ [๐ฎโƒ—+๐ฏโƒ—]โˆˆ ๐•Œโฟ
  • Linear: [๐›ผ๐ฎโƒ—+๐›ฝ๐ฏโƒ—]โˆˆ ๐•Œโฟ
    • ๐›ผ,๐›ฝโˆˆ๐”ธ

Region

๐•Šโฟ โŠ‚ ๐•Œโฟ

Subspace ๐•Šโฟ: Space conditions

Sphere

Equation of a Sphere.

  • Centre: $๐–ข(h,k,l)$
  • Point: $๐–ฏ(x,y,z)$
$$\color{Cyan} (x-h)ยฒ + (y-k)ยฒ + (z-l)ยฒ = rยฒ$$ $$\color{Cyan} |๐–ฏ-๐–ข| = r โ €โ €โ €๐–ฏ,๐–ขโˆˆโ„โฟ ; rโ‰ฅ0 โˆˆโ„$$

Straight Line

Given a Point $\color{BlueGreen} ๐–ฏ(x_0,y_0,z_0)$ and a vector $\color{CornflowerBlue}๐ฏโƒ— = โŸจv_x, v_y, v_zโŸฉ$ a line can be described by the equations:

$$\left\{\begin{matrix} x = x_0 + v_xยทt \\\ y = y_0 + v_yยทt \\\ z = z_0 + v_zยทt \end{matrix}\right.$$

Or in general

$$\color{Cyan} โ„’โƒ— = {\color{BlueGreen} ๐–ฏ} + {\color{silver}t} ยท {\color{CornflowerBlue}๐ฏโƒ—}$$

General Conic Equation

$$\color{Emerald} Aยทx^2 + Cยทy^2 + Dx + Ey + F = 0$$
  • $A = 0$ OR $C=0$
    • Parabola
  • $A = C$
    • Circunference
  • $sign(A) = sign(C)$
    • Elipse
  • $sign(A) = -sign(C)$
    • Hiperbola
$$\color{Emerald} Aยทx^2 + Bxy + Cยทy^2 + Dx + Ey + F = 0$$
  • $B^2 - 4AC = 0$
    • Parabola
  • $B^2 - 4AC < 0$
    • Elipse
  • $B^2 - 4AC > 0$
    • Hiperbola

With $\color{Cerulean}๐–ฅ$ the fixed point, Focus, $\color{NavyBlue}๐“$ the straight line, Directrix, and $\color{RedOrange}โ„ฏ$ a factor called Excentricity, then the set of all points $\color{ProcessBlue}๐‘ƒ$ such as

$$\color{Cyan} \frac{ distance({\color{ProcessBlue}๐‘ƒ}, {\color{Cerulean}๐–ฅ}) }{ distance({\color{ProcessBlue}๐‘ƒ}, {\color{NavyBlue}๐“}) } = {\color{RedOrange}โ„ฏ}$$

With:

  • ${\color{RedOrange}โ„ฏ} = 1$ for Parรกbolas
  • ${\color{RedOrange}โ„ฏ} < 1$ for Elipses
  • ${\color{RedOrange}โ„ฏ} > 1$ for Hiperbola

Plane

Given a Point $\color{BlueGreen} ๐–ฏ_0(x_0,y_0,z_0)$ of the plane and the Normal Vector to the surface $\color{LightBlue}๐งโƒ— = โŸจn_x, n_y, n_zโŸฉ$ the Plane is defined by

$$\color{SkyBlue} a(x-x_0) + b(y-y_0) + c(z-z_0) = 0$$

If we call the Position Point $\color{ProcessBlue}๐–ท(x,y,z)$ :

$$\color{SkyBlue} { \color{LightBlue}๐งโƒ— } ยท ( {\color{ProcessBlue}๐–ท} - {\color{BlueGreen} ๐–ฏ_0} ) = 0$$

Plane from three points

Having three points

$$\vec{A}, \vec{B}, \vec{C}$$

we can calculate the Normal Vector to the plane

$$\begin{matrix} \vec{u} = \vec{B} - \vec{A} \\\ \vec{v} = \vec{C} - \vec{A} \\\ then \\\ \vec{N} = \vec{u} x \vec{v} \end{matrix}$$

Equation of a Plane in Space

The equation of the plane through $(x_0, y_0, z_0)$ that has a normal vector

$\vec{n} = A\hat{x} + B\hat{y} + C \hat{z} .$ is:

$$A (x - x_0) + B (y - y_0) + C(z - z_0) = 0$$

transformable to the system:

$$Ax + By + Cz+ D = 0 \text{ where } D=-Ax_0-By_0- Cz_0.$$

That we can generalize as

$$\color{silver} \vec{n}ยท(\vec{x} - \vec{P})$$
%%{init: {"quadrantChart": {"xAxisPosition": "bottom", "yAxisPosition": "left", "xRange": [-10, 10], "yRange": [-10, 10]}}} "titlePadding": 20}}}%%

quadrantChart
    title Point(7,6) and N(4, 2.3)
    x-axis x
    y-axis y
    
    X: [0.7, 0.6]
    N: [0.4, 0.23]

    S7_6: [0.0, 0.0] color: #0706FF

    S7_7: [0.0, 0.22999999999999998] color: #0707FF

    S7_8: [0.0, 0.45999999999999996] color: #0708FF

    S7_9: [0.0, 0.69] color: #0709FF

    S7_10: [0.0, 0.9199999999999999] color: #0710FF

    S8_6: [0.4, 0.0] color: #0806FF

    S8_7: [0.4, 0.22999999999999998] color: #0807FF

    S8_8: [0.4, 0.45999999999999996] color: #0808FF

    S8_9: [0.4, 0.69] color: #0809FF

    S8_10: [0.4, 0.9199999999999999] color: #0810FF

    S9_6: [0.8, 0.0] color: #0906FF

    S9_7: [0.8, 0.22999999999999998] color: #0907FF

    S9_8: [0.8, 0.45999999999999996] color: #0908FF

    S9_9: [0.8, 0.69] color: #0909FF

    S9_10: [0.8, 0.9199999999999999] color: #0910FF



Loading
scale = 10
(x0, y0) = (7, 6) # Point in Surface
(a, b) = (4, 2.3) # Normal
S = 10 # Origin Line Points
dS = 1/S # line segment size

for x in range(0,S+1):
    for y in range(0,S+1):
        X = a*(x - x0 )/scale
        Y = b*(y - y0 )/scale
        if X<= 1 and X>=0 and Y<= 1 and Y>=0:
            print(f"    S{x}_{y}: [{X }, {Y }] color: #{x:02}{y:02}FF\n")

(Signed) Distance from a point $(p)$ to a Plane $(S)$

$$\color{gold} d = \hat{n}ยท(\vec{P} - \vec{S_0})$$
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