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--- data_mining:neural_network:perceptron [2017/02/02 22:04] – [Weight space] phreazer
+++ data_mining:neural_network:perceptron [2017/02/03 22:21] (current) – [What perceptrons can't do] phreazer
@@ Line 17: / Line 17: @@
 ====== Geometrical Interpretation ======
-===== Weight space =====
+===== Weight-Space view =====
-  * 1 dimensions for each weight
+  * 1 dimension for each weight
-  * Point represents setting of all weights
+  * Point represents a setting of all weights
-  * Leaving the threshold out, each **training case** can be represented as a **hyperplane** through the **origin**.
+  * Leaving the threshold out, each **training case** can be represented as a **hyperplane** through the **origin**. Inputs represent planes (or Constraints)
      * For a particular training case: Weights must lie on one side of this hyper-plane to get the answer correct.
-Plane is perpendicular to the input vector. Good weight vector needs to be on the same side of the hyperplane.
-Scalar product of wight vector and input vector positiv (angle <= 90°).
+Plane goes through **origin**, is perpendicular to the input vector (with correct answer = 1 (or 0)). Good weight vector needs to be on the same side of the hyperplane.
+Scalar product of wight vector and input vector positiv (angle < 90°).
+**Cone of feasable solutions**
+Need to find a point on right side of all the planes (training cases): Might not exist.
+If there are weight vectors that get the right side for all cases, they lie in a hyper-cone, with apex in origin.
+Average of two good wight vectors is a good weight vector => Convex problem.
+==== Why learning works ====
+Proof that squared distance between feasable and current weight vector get's smaller. Hopeful claim: Every time perceptron makes mistakes, the current weight vector gets closer to all feasble wight vectors. Not true.
+"generously feasable" weight vector, that lie within a feasible region by a **margin** at least as great as the **length of the input vector** that defines **each constraint plane**.
+Every time perceptron makes mistake, squared distance to all of the **generously feasible** weight vectors is always decreased by at least the **squared length** of the **update vector**.
+After finite number of mistakes, the weight vector must lie in the **feasible** region, if the region exists.
+==== What perceptrons can't do ====
+Separate feature unit for each of the many binary vectors => any possible discrimination. Exponential many features are necessary.
+This type of table look-up won't generalize. (guess due to the binary encoding).
+Binary threshold output unit can't tell if two single bit features are the same.
+=> contradicting constraints.
+=== Data-Space ===
+- Each input vector is point in space
+- Weight vector defines plane
+- Weight plane is perpendicular to the weight vector and misses the origin by a distance equal to the threshold.
+Pos and neg cases can not be seperated by a plane => not linearly separable.
+=== Discriminate patterns under translation with wrap-around ===
+Binary threshold neuron can't discriminate between different patterns that have same number of on pixels (if pattern can translate with wrap-around).
+Proof:
+Pattern A on all possible translations (4 on pixels).
+Total Input will be 4x sum of all the weights.
+Pattern B on all possible translations (4 on pixels)
+Total Input will be 4x sum of all the weights.
+But every single case of pattern A must provide more import than all single cases of pattern B.
+Case:
+Patterns with 2 Classes.
+First class contains patterns with 4 pixels on.
+Second class contains pattern with either 1 or 3 pixels on.
+Weight from each pixel = 1.
+Bias = -3.5
+Any example with 3 pixels on: activation = -0.5
+Any example with 1 pixel on: activation = -2.5
+Any example with 4 pixel on: acitvation = 0.5
+=> correctly classified.
+Whole point of pattern recognition is to recognize patterns despite transformations like translation. Minksy and Paper ("Group Invariance Theorem"): Part of perceptron that **learns** cannot learn to do this if the transformation form a **group**:
+- Translation with **wrap-around** form a group.
+To deal with such transf. tricky part of recognition must be solved by hand-coded **feature detectors** (not learning procedure).
+Conclusion (after 20 yrs): Neuronal Network has to learn the feature detectors (not only weights).
+  * More layers of linear units do not help (still linear).
+  * Fixed output non-linearities are not enough.
+=> Need multiple layers of **adaptive**, **non-linear** hidden units.
+  * Efficient way of adapting all the weights.
+  * Learning weights going into hidden units is equivalent to learning features.
+  * No one is telling us what the feature vectors should be.