Machine Learning Foundations - week2 key point
2017-09-02 10:48
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1. when can machines learn?
1.2 Learning to answer yes/no
PLAA takes linear separable D and perceptrons H to get hypothesisg
unknown target function (f: x --> y)
training examplesD: (x1, y1), ···, (xn, yn) ---> learning algorithm A ---> final hypothesis g ≈ f
(hypothesis set H, H = all possible perceptrons)
Perceptron:
A Simple Hypothesis Set: the ‘Perceptron’: called ‘perceptron’ hypothesis historically
Vector Form of Perceptron Hypothesis: h(x) = sign(w^Tx)
Perceptrons in R^2: perceptrons ⇔ linear (binary) classifiers
Perceptron Learning Algorithm (PLA):
start from some w0 (say, 0), and ‘correct’ its mistakes on D
Linear Separability:
linear separable D ⇔ PLA halts (i.e. no more mistakes)⇔ exists perfect wf such
that yn = sign(wf^T xn)
<
4000
/p>
More about PLA:
As long as linear separable and correct by mistake
• inner product of wf and wt grows fast; length of wt grows slowly (T
<= (R/p)^2)
• PLA ‘lines’ are more and more aligned with wf ⇒ halts
Learning with Noisy Data --> Pocket Algorithm:
modify PLA algorithm (black lines) by keeping best weights in pocket
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