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CIS 541 – Numerical MethodsPowerPoint Presentation

CIS 541 – Numerical Methods

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### CIS 541 – Numerical Methods

Mathematical Preliminaries

f(x)

t

Tangents and Gradients- Recall that the slope of a curve (defined as a 1D function) at any point x, is the first derivative of the function.
- That is, the linear approximation to the curve in the neighborhood of t is l(x) = b + f’(t)x

y

x

OSU/CIS 541

Tangents and Gradients

- Since we also want this linear approximation to intersect the curve at the point t.
l(t) = f(t) = b + f’(t)t

- Or, b = f(t) - f’(t)t
- We say that the line l(x)interpolates the curve f(x) at the point t.

OSU/CIS 541

Functions as curves

- We can think of the curve shown in the previous slide as the set of all points (x,f(x)).
- Then, the tangent vector at any point along the curve is

OSU/CIS 541

Side note on Curves

- There are other ways to represent curves, rather than explicitly.
- Functions are a subset of curves (x,y(x)).
- Parametric equations represent the curve by the distance walked along the curve (x(t),y(t)).
Circle: (cos, sin)

- Implicit representations define a contour or level-set of the function: f(x,y) = c.

OSU/CIS 541

Tangent Planes and Gradients

- In higher-dimensions, we have the same thing:
- A surface is a 2D function in 3D:
Surface = (x, y, f(x,y) )

- A volume or hyper-surface is a 3D function in 4D:
Volume = (x, y, z, f(x,y,z) )

OSU/CIS 541

Tangent Planes and Gradients

- The linear approximation to the higher-dimensional function at a point (s,t), has the form: ax+by+cz+d=0, or z(x,y) = …
- What is this plane?

OSU/CIS 541

Construction of Tangent Planes

Images courtesy of TJ Murphy: http://www.math.ou.edu/~tjmurphy/Teaching/2443/TangentPlane/TangentPlane.html

OSU/CIS 541

Construction of Tangent Planes

OSU/CIS 541

Construction of Tangent Planes

OSU/CIS 541

Tangent Planes and Gradients Of course, the plane does not stay close to the surface as you move away from the point (s,t).

- The formula for the plane is rather simple:
- z(s,t) = f(s,t) - interpolates
- z(s+dx,t) = f(s,t) + fx(s,t)dx = b + adx
- Linear in dx

OSU/CIS 541

Tangent Planes and Gradients

- The normal to the plane is thus:
- The 2D vector:is called the gradient of the function.
- It represents the direction of maximal change.

OSU/CIS 541

Gradients

- The gradient thus indicates the direction to walk to get down the hill the fastest.
- Also used in graphics to determine illumination.

OSU/CIS 541

Review of Functions

- Extrema of a function occur where f’(x)=0.
- The second derivative determines whether the point is a minimum or maximum.
- The second derivative also gives us an indication of the curvature of the curve. That is, how fast it is oscillating or turning.

OSU/CIS 541

The Class of Polynomials

- For many polynomials, the latter coefficients are zero. For example:
p(x) = 3+x2+5x3

OSU/CIS 541

Taylor’s Theorem

- Some things to note:
- (x-c)(n+1) quickly approaches zero if |x-c|<<1
- (x-c)(n+1) increases quickly if |x-c|>>1
- Higher-order derivatives may get smaller (for smooth functions).

OSU/CIS 541

Higher Derivatives

- What is the 100th derivative of sin(x)?
- What is the 100th derivative of sin(3x)?
- Compare 3100 to 100!

- What is the 100th derivative of sin(1000x)?

OSU/CIS 541

Taylor’s Theorem

- Hence, for points near c we can just drop the error term and we have a good polynomial approximation to the function (again, for points near c).
- Consider the case where (x-c)=0.5
- For n=4, this leads to an error term around 2.6*10-4 f()
- Do this for other values of n.
- Do this for the case (x-c) = 0.1

OSU/CIS 541

Some Common Derivatives

OSU/CIS 541

Book’s Introduction Example

- Eight terms for first series not even yielding a single significant digit.
- Only four for second serieswith foursignificantdigits.

OSU/CIS 541

Mean-Value Theorem

- Special case of Taylor’s Theorem, where n=0, x=b.
- Assumes f(x) is continuous and its first derivative exists everywhere within (a,b).

OSU/CIS 541

Mean-Value Theorem

- So what!?!?! What does this mean?
- Function can not jump away from current value faster than the derivative will allow.

f(x)

a

secant

b

OSU/CIS 541

Rolles Theorem

- If a and b are roots (f(a)=f(b)=0) of a continuous function f(x), which is not everywhere equal to zero, then f’(t)=0 for some point t in (a,b).
- I.e., What goes up, must come down.

f(x)

f’(t)=0

a

t

b

OSU/CIS 541

Caveat

- For Taylor’s Series and Taylor’s Theorem to hold, the function and its derivatives must exist within the range you are trying to use it.
- That is, the function does not go to infinity, or have a discontinuity (implies f’(x) does not exist), …

OSU/CIS 541

Implementing a Fast sin()

const int Max_Iters = 100,000,000;

float x = -0.1;

float delta = 0.2 / Max_Iters;

float Reimann_sum = 0.0;

for (int i=0; i<Max_Iters; i++)

{

Reimann_Sum += sinf(x);

x+=delta;

}

Printf(“Integral of sin(x) from –0.1->0.1 equals: %f\n”, Reimann_Sum*delta );

OSU/CIS 541

Implementing a Fast sin()

const int Max_Iters = 100,000,000;

float x = -0.1;

float delta = 0.2 / Max_Iters;

float Reimann_sum = 0.0;

for (int i=0; i<Max_Iters; i++)

{

Reimann_Sum += my_sin(x); //my own sine func

x+=delta;

}

Printf(“Integral of sin(x) from –0.1->0.1 equals: %f\n”, Reimann_Sum*delta );

OSU/CIS 541

Version 1.0

my_sin( const float x )

{

float x2 = x*x;

float x3 = x*x2;

return (x – x3/6.0 + x2*x3/120.0 );

}

OSU/CIS 541

Version 2.0 – Horner’s Rule

Static const float fac3inv = 1.0 / 6.0f;

Static const float fac5inv = 1.0 / 120.0f;

my_sin( const float x )

{

float x2 = x*x;

return x*(1.0 – x2*(fac3inv - x2*fac5inv));

}

OSU/CIS 541

Version 3.0 – Inline code

const int Max_Iters = 100,000,000;

float x = -0.1;

float delta = 0.2 / Max_Iters;

float Reimann_sum = 0.0;

for (i=0; i<Max_Iters; i++)

{

x2 = x*x;

Reimann_Sum += x*(1.0–x2*(fac3inv-x2*fac5inv);

x+=delta;

}

Printf(“Integral of sin(x) from –0.1->0.1 equals: %f\n”, Reimann_Sum*delta );

OSU/CIS 541

Observations

- Is the result correct?
- Why did we gain some accuracy with version 2.0?
- Is (–0.1,0.1) a fair range to consider?
- Is the original sinf() function optimized?
- How did we achieve our speed-ups?
- We will re-examine this after Lab1.

Ask these

question for

Lab1 !!!

OSU/CIS 541

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