Polynomials. 2 Content Evaluation Root finding Root Bracketing Interpolation Resultant.
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Transcript of Polynomials. 2 Content Evaluation Root finding Root Bracketing Interpolation Resultant.
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Polynomials
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Content
EvaluationRoot findingRoot BracketingInterpolationResultant
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IntroductionBest understood and most applied functions
Taylor’s expansion Basis of parametric curve/surface Data fitting
Basic Theorem: Weierstrass Approximation Theorem Fundamental Theorem of Algebra
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Evaluation – Horner’s Method
Compare number of multiplications and additions
01321)( axaaxaxaxaxp nnnn
Evaluate p(t)
p(t) = b0
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Details
3210
33
2210
)(
)(
taatatatp
xaxaxaaxp
100
211
322
33
)(
tbabtp
tbab
tbab
ab
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Details (cont)
)()(
)()()(
)(
)(
0
122
30
10212
323
3
012
23
3
tqtp
xqtxxqxp
xqtxb
btxxbtxxbtxb
tbbxtbbxtbbxb
axaxaxaxp
122
3)( bxbxbxq
100
211
322
33
)(
tbabtp
tbab
tbab
ab
Evaluate p(t) and p’(t)
p(t) = b0
p’(t) = c1
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Evaluating xk Efficiently
Instead of using pow(x,k), or any iterative/recursive subroutines, think again!The S-and-X method: S(square) X(multiply-by-x)See how it works in the next pagesUnderstand how to implement in program
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Say the exponent k in base-2: a2a1a0 (k>3)
7
12121
22
]][[
7111
x
x
xxx
SXSX
6
02121
22
]][[
6110
x
x
xx
SSX
5
12021
22
]][[
5101
x
x
xx
SXS
Except for the leading digit, replace 1 by [SX], 0 by [S]
Except for the leading digit, replace 1 by [SX], 0 by [S]
# of :
Each symbol, S or X, represents a multiplication
23
12121221
2222
]][][][[
2310111
x
x
xxxx
SXSXSXS
7 4 3 3
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ImplementationLeft-to-right scan
23
121212021
2222
]][][][[
2310111
x
x
xxxx
SXSXSXS
11101x2x4x16 x8 xz
Right-to-left scan
2321125222
120
1
1
1
0
1
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[Multiplication, division]
FFT…GCD of polynomials (Euler algorithm)
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Roots of Polynomials
Low order polynomials (for degree 4)
Quartics: see notes
Hi-precision formula for quadratics
Quadratics Cubics
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Calculator City (ref)
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Root Counting
P(x) has n complex roots, counting multiplicitiesIf ai’s are all real, then the complex roots occur in conjugate pairs.Descarte’s rules of sign Sturm’s sequenceBounds on roots[Deflation]
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Theorems for Polynomial Equations
Sturm theorem: The number of real roots of an
algebraic equation with real coefficients whose real roots are simple over an interval, the endpoints of which are not roots, is equal to the difference between the number of sign changes of the Sturm chains formed for the interval ends.
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Sturm Chain
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Example
38879.1 ,32836.10802951.0 ,334734.0 ,21465.1
13)( 5
ix
xxxf
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Sturm Theorem (cont)
For roots with multiplicity: The theorem does not apply, but … The new equation : f(x)/gcd(f(x),f’(x))
All roots are simple All roots are same as f(x)
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Sturm Chain by Maxima
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Maxima (cont)
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Descarte’s Sign RuleA method of determining the maximum number of positive and negative real roots of a polynomial. For positive roots, start with the sign of the coefficient of the lowest power. Count the number of sign changes n as you proceed from the lowest to the highest power (ignoring powers which do not appear). Then n is the maximum number of positive roots.
For negative roots, starting with a polynomial f(x), write a new polynomial f(-x) with the signs of all odd powers reversed, while leaving the signs of the even powers unchanged. Then proceed as before to count the number of sign changes n. Then n is the maximum number of
negative roots.
3 positive roots
4 negative roots
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Computing Roots Numerically
Newton is the main method for root findingCan be implemented efficiently using Horner’s methodQuadratic convergence except for multiple rootUse deflation to resolve root multiplicity; but can accumulate error; polynomials are sensitive to coefficient variationsNewton-Maehly’s method: roots converges quadratically even if previous roots are inaccurate
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Experiment
P(x)=(x-1)(x-2)(x-3)(x2+1)Assume two imprecise roots have been found, 1.1 & 1.9The deflated (cubic) polynomial is then
x3-3x2+0.91x-3
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Maehly procedureFast convergence; accurate solution
Deflated cubicFaster convergence, but solution was plagued with the propagated error
Original quinticConverged to 3in many steps (5 was not a good guess for 3)
9.11
1.111 )()(
)(
kk xxkk
kkk xpxp
xpxx
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Polynomial Interpolation
Given (n+1) pairs of points (xi,f(xi)) find a nth-degree polynomial Pn(x) to pass through these pointsCompute the function value not listed in the table by evaluating the interpolating polynomial
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Lagrange Polynomial
High computational cost; cannot reuse pointsDivided difference: a better alternative
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Divided Difference
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Divided Difference (cont)
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Resultant (of two polynomials)
An expression involving polynomial coefficients such that the vanishing of the expression is necessary and sufficient for these polynomials to have a common zero.
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Resultant (cont)
The equation Qz = 0 has nonzero solution IFF R=det(Q) vanishes.R is called the resultant of the equations.
with
The above system has common zero:
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Example
0
341
341
211
211
034
022
2
R
xx
xx
8
341
341
211
211
034
022
2
R
xx
xx
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Sylvester Matrix and Resultant
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Applications of Resultant
0)(
0)(
xq
xp
0),(
0),(
yxq
yxp
0)(
0)(
curve same for the 0),(
equation ingcorrespond Find
)(),()( :curve
ytq
xtp
yxF
implicit
tqtptcParamateri
R(x) = 0 IFF intersection exists
II. Algebraic curve intersection
I. Common zero
III. Implicitization
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Find Common Zero
The system has simultaneous zero iff the resultant is zero
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Result from Linear Algebra
has non-trivial solution if det(A) = 0
A reduced system (remove row n) gives the ratio:
Ai: remove column i
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Sylvester Resultant
To find the common zero, consider the reduced system
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Resultant in Maxima
x as independent variable
Byy
Byy
Ay
R
yxyB
Ayx
2
2
2
2
0
0
101
0
01
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Algebraic Curve Intersection
01),(
01),( 22
xyyxq
yxyxp
No real roots; no intersection (circle & hyperbola)
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Algebraic Curve Intersection
01),(
011),( 22
xyyxq
yxyxp
2 real roots; 2 intersection points
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Implicitization
The above system in t has a common zero whenever (x,y) is a point on the curve.
… a parabola
Application: parametric curve intersectionFind the corresponding parameter of (x,y) on the curve
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Parametric Curve Intersection
Cubic Bezier curve and its control points
Bernstein polynomial
implicitize f(x,y)=0 )(),()(:1 tytxtpc
)(),()(:2 sysxsqc
F(s)=f(x(s),y(s))=0
Find roots in [0,1]
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0012
210
0012
210
221
20
221
20
2
2
2
1
12
0
0
222
222
121
121
121)(
)()(
ytyytyyy
xtxxtxxx
tyttyty
txttxtx
ty
xtt
y
xt
y
x
ty
txtp
12
2)(:
1
2,
1
1,
1
0:)(
23
2)(:
1
2,
1
1,
0
0:)(
222
211
s
sscsc
tt
ttctc
29.0,69.0
04820
01242423
0443),(
2
22
21
s
ss
sss
yxxyxf
Quadratic Bezier Curve Intersection
Sketch using de Casteljau algorithm
[1, 2]
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De Casteljau Algorithm [ref]
A cubic Bezier curve with 4 control points
p(t), t[0,1] is defined.
Locate p(0.5)
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Inversion
41
419
21
,,
yx
t
2
1
05
10
05
/
01
15
01
0
1015
150
015
:rowfourth theDiscarding
0
115
0115
15
0315
43
47
47
43
47
47
2
3
2
3
43
47
47
2
3
43
41
47
419
t
tt
t
t
t
t
t
t
Example
Given (x,y) on the curve, find the corresponding parameter t