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+ | =Lecture 27, [[ECE662]]: Decision Theory= | ||
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+ | Lecture notes for [[ECE662:BoutinSpring08_Old_Kiwi|ECE662 Spring 2008]], Prof. [[user:mboutin|Boutin]]. | ||
+ | |||
+ | Other lectures: [[Lecture 1 - Introduction_Old Kiwi|1]], | ||
+ | [[Lecture 2 - Decision Hypersurfaces_Old Kiwi|2]], | ||
+ | [[Lecture 3 - Bayes classification_Old Kiwi|3]], | ||
+ | [[Lecture 4 - Bayes Classification_Old Kiwi|4]], | ||
+ | [[Lecture 5 - Discriminant Functions_Old Kiwi|5]], | ||
+ | [[Lecture 6 - Discriminant Functions_Old Kiwi|6]], | ||
+ | [[Lecture 7 - MLE and BPE_Old Kiwi|7]], | ||
+ | [[Lecture 8 - MLE, BPE and Linear Discriminant Functions_Old Kiwi|8]], | ||
+ | [[Lecture 9 - Linear Discriminant Functions_Old Kiwi|9]], | ||
+ | [[Lecture 10 - Batch Perceptron and Fisher Linear Discriminant_Old Kiwi|10]], | ||
+ | [[Lecture 11 - Fischer's Linear Discriminant again_Old Kiwi|11]], | ||
+ | [[Lecture 12 - Support Vector Machine and Quadratic Optimization Problem_Old Kiwi|12]], | ||
+ | [[Lecture 13 - Kernel function for SVMs and ANNs introduction_Old Kiwi|13]], | ||
+ | [[Lecture 14 - ANNs, Non-parametric Density Estimation (Parzen Window)_Old Kiwi|14]], | ||
+ | [[Lecture 15 - Parzen Window Method_Old Kiwi|15]], | ||
+ | [[Lecture 16 - Parzen Window Method and K-nearest Neighbor Density Estimate_Old Kiwi|16]], | ||
+ | [[Lecture 17 - Nearest Neighbors Clarification Rule and Metrics_Old Kiwi|17]], | ||
+ | [[Lecture 18 - Nearest Neighbors Clarification Rule and Metrics(Continued)_Old Kiwi|18]], | ||
+ | [[Lecture 19 - Nearest Neighbor Error Rates_Old Kiwi|19]], | ||
+ | [[Lecture 20 - Density Estimation using Series Expansion and Decision Trees_Old Kiwi|20]], | ||
+ | [[Lecture 21 - Decision Trees(Continued)_Old Kiwi|21]], | ||
+ | [[Lecture 22 - Decision Trees and Clustering_Old Kiwi|22]], | ||
+ | [[Lecture 23 - Spanning Trees_Old Kiwi|23]], | ||
+ | [[Lecture 24 - Clustering and Hierarchical Clustering_Old Kiwi|24]], | ||
+ | [[Lecture 25 - Clustering Algorithms_Old Kiwi|25]], | ||
+ | [[Lecture 26 - Statistical Clustering Methods_Old Kiwi|26]], | ||
+ | [[Lecture 27 - Clustering by finding valleys of densities_Old Kiwi|27]], | ||
+ | [[Lecture 28 - Final lecture_Old Kiwi|28]], | ||
+ | ---- | ||
+ | ---- | ||
+ | |||
== Clustering by finding valleys of densities == | == Clustering by finding valleys of densities == | ||
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<math>u(1)=b \Rightarrow c_1 + c_2 = b \Rightarrow c_1=b-a</math> (3-17) | <math>u(1)=b \Rightarrow c_1 + c_2 = b \Rightarrow c_1=b-a</math> (3-17) | ||
− | + | [[Image:park212_Old Kiwi.jpg]] | |
In general, cannot solve Euler equation analytically | In general, cannot solve Euler equation analytically | ||
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Pick initial curve <math>u_0 (x)</math> | Pick initial curve <math>u_0 (x)</math> | ||
− | Consider a family of curves <math>u(t,x)</math> such that <math>u(0,x)=u_0 (x)</math> and <math>\frac{\partial u}{\partial t}=E'</math> (3-18) | + | Consider a family of curves <math>u(t,x)</math> such that <math>u(0,x)=u_0 (x)</math> and <math>\frac{\partial u}{\partial t}= \pm E'</math> (3-18) |
+: if looking for max | +: if looking for max | ||
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<math>u(t+\Delta t, x)=u(t,x)+\frac{\Delta t}{\Delta x^2}(u(t,x+\Delta x)-2u(t,x)+u(t,x-\Delta x))</math> (3-30) | <math>u(t+\Delta t, x)=u(t,x)+\frac{\Delta t}{\Delta x^2}(u(t,x+\Delta x)-2u(t,x)+u(t,x-\Delta x))</math> (3-30) | ||
+ | ---- | ||
+ | Previous: [[Lecture_26_-_Statistical_Clustering_Methods_Old_Kiwi|Lecture 26]] | ||
+ | Next: [[Lecture_28_-_Final_lecture_Old_Kiwi|Lecture 28-Final lecture]] | ||
+ | |||
+ | [[ECE662:BoutinSpring08_OldKiwi|Back to ECE662 Spring 2008 Prof. Boutin]] | ||
+ | [[Category:ECE662]] | ||
+ | [[Category:decision theory]] | ||
+ | [[Category:lecture notes]] |
Latest revision as of 08:44, 17 January 2013
Contents
Lecture 27, ECE662: Decision Theory
Lecture notes for ECE662 Spring 2008, Prof. Boutin.
Other lectures: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28,
Clustering by finding valleys of densities
-- Approach 1: "Bump Hunting" --
- Approximate p(x) - Find peaks of density - Expand cluster boundaries outward until they meet (hopefully in middle!) - Define meeting points = Cluster boundary points
-- Approach 2: "Valley Seeking" --
- Start from valley regions of p(x) - Work uphill to connect data points to density peaks
Graph based implementation
- Estimate p(x) - For all patterns $ X_i $, denote by $ N_r(X_i) $, the neighborhood of size $ r $ of $ X_i $. - Estimate derivative of density p(x) along direction $ X_i-X_j, \forall X_j\in N_r(X_i) $ $ S_i(j)=\frac{p(X_i)-p(X_j)}{||X_i-X_j||_{L_2}} $ Let $ j_{max}=argmax_j S_i(j) $ If $ S_i(j_{max})<0 $ declare $ i $ to be a root node. If $ S_i(j_{max})>0 $ declare $ X_{j_{max}} $ to be parent of $ X_i $. If $ S_i(j_{max})=0 $ it depends on the surroundings what link we establish. - If $ X_i $ is connected to a root node, then make this root node a parent of $ X_i $, else make $ X_i $ a root node. - Repeat
When all points are visited, a forest is constructed. Each connected component (=tree of a forest) is a cluster.
- Problem: All methods presented above encounter a serious problem because of the following reasons:
- Density may oscillate a lot - Valleys are not well defined.
PDE based valley seeking
PDE: Partial Differential Equation
PDE's can be used to minimize energy functionals
- Simple Example
Consider 1-D curves
$ u(x): [0,1] \rightarrow \Re $ with fixed end points $ u(0)=a $, $ u(1)=b $ (3-1)
Suppose Energy of curve is $ E(u)=\int _0 ^1 F(u, u')dx $ for some function $ F: \Re ^2 \rightarrow \Re $
e.g.) $ F(u,u')=|u'|^2 $ (3-2)
The curve that minimizes (or maximizes) E(u) satisfies Euler Equation
$ \frac{\partial F} {\partial u} -\frac{d}{dx}(\frac{\partial F}{\partial u'})=0 $ (3-3)
sometimes written as $ E'=0 \Rightarrow \frac{\partial E}{\partial u}=0 $ (3-4)
Similarly if $ E=\int _0 ^1 F(u,u',u'')dx $ (3-5)
e.g.) $ F(u,u',u'')=|u''|^2 $ (3-6)
Then Euler equation is $ \frac{\partial F}{\partial u} - \frac{d}{dx}(\frac{\partial F}{\partial u'}) + \frac{d^2}{dx^2}(\frac{\partial F}{\partial u''})=0 $ (3-7)
Similarly, for surface in $ \Re ^2 $
$ u(x,y): [0,1] \times [0,1] \rightarrow \Re $ (3-8)
Suppose energy is given by
$ E(u)=\int _{surface} F(u,u_x, u_y, u_{xx}, u_{xy},u_{yy})dxdy $ (3-9)
e.g.) $ F={u_x}^2+{u_y}^2 $ (3-10)
Then Euler equation is
$ \frac{\partial F}{\partial u} - \frac{d}{dx}(\frac{\partial F}{\partial u_x}) - \frac{d}{dy}(\frac{\partial F}{\partial u_y}) + \frac{d^2}{dx^2}(\frac{\partial F}{\partial u_{xx}}) + \frac{d^2}{dy^2}(\frac{\partial F}{\partial u_{yy}}) + \frac{d^2}{dxdy}(\frac{\partial F}{\partial u_{xy}})=0 $ (3-11)
- Illustration
1D curve $ u(x) $ with $ u(0)=a, u(1)=b $ (3-12)
$ E(u)=\int _{0} ^{1} ||\nabla u||^2 dx $ (3-13) Dirichlet integral
$ F=||\nabla u||^2=u_x ^2 $
Euler equation E'=0
$ \frac{\partial F}{\partial u} - \frac{d}{dx}(\frac{\partial F}{\partial u_x})=0 $ (3-14)
$ 0-\frac{d}{dx}(2u_x)=0 \Rightarrow -2u_{xx}=0 \Rightarrow u_{xx}=0 $ (3-15)
with boundary condition, $ u(0)=0, u(1)=b $
Solution: $ u(x)=c_1 x+ c_2 $
$ u(0)=a \Rightarrow c_2=a $ (3-16)
$ u(1)=b \Rightarrow c_1 + c_2 = b \Rightarrow c_1=b-a $ (3-17)
In general, cannot solve Euler equation analytically
In practice, use 'Gradient descent'
Pick initial curve $ u_0 (x) $
Consider a family of curves $ u(t,x) $ such that $ u(0,x)=u_0 (x) $ and $ \frac{\partial u}{\partial t}= \pm E' $ (3-18)
+: if looking for max
-: if looking for min
t is called "time marching parameter"
Solve for $ u(t,x) $ numerically, when "steady state" $ \frac{\partial u}{\partial E}=0 $ is achieved. (3-19)
Say at $ u(t_{final},x)=: u_1 (x) $ (3-20)
Then $ u_1(x) $ is the solution to Euler Equation $ E'=0 $
- Example
$ F=||\nabla u||^2 = u_x ^2 $ (3-21)
Look for curve $ u(x) $ such that $ u(0)=a, u(1)=b $ with minimum E. (3-22)
Consider $ u(t,x) $
$ u(0,x) $ is initial guess
Gradient descent (Equation to Euler's equation)
$ \frac{\partial u}{\partial t}=u_{xx} $ "Heat Equation" (3-23)
<<Picture>>
- curve gets less and less curvy as t evolves
- in fact, evolving for t units is equivalent to convolving $ u_0 (x) $ with Gaussian kernel
$ G(x,t)=\sqrt{\frac{1}{4 \pi t}} e^{-\frac{x^2}{4t}} $ (3-24)
i.e., $ u(t,x)=u_0 (x) * G(x,t) $
<<Figure>>
Scale space of curves
If cannot solve $ \frac{\partial u}{\partial t}=E' $ discretize and solve numerically (3-25)
$ \frac{\partial u}{\partial t} \approx \frac{u(t+ \Delta t ,x )-u(t,x)}{\Delta t} $ (3-26)
$ u_{xx} \approx \frac{u(t,x+\Delta x)-2u(t,x)+u(t,x-\Delta x)}{\Delta x^2} $ (3-27)
$ \frac{\partial u}{\partial t}=u_{xx} $ (3-28)
$ \frac{u(t+\Delta t,x)-u(t,x)}{\Delta t}=\frac{u(t,x+\Delta x)-2u(t,x)+u(t,x-\Delta x)}{\Delta x^2} $ (3-29)
$ u(t+\Delta t, x)=u(t,x)+\frac{\Delta t}{\Delta x^2}(u(t,x+\Delta x)-2u(t,x)+u(t,x-\Delta x)) $ (3-30)
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