## Backpropagation

• The divergence function minimized is only a proxy for classification error(like Softmax)
• Minimizing divergence may not minimize classification error
• Does not separate the points even though the points are linearly separable
• This is because the separating solution is not a feasible optimum for the loss function
• Compare to perceptron
• Perceptron rule has low bias(makes no errors if possible)
• But high variance(swings wildly in response to small changes to input)
• Backprop is minimally changed by new training instances
• Prefers consistency over perfection(which is good)

## Convergence

### Univariate inputs

$\text {Minimize } E=\frac{1}{2} a w^{2}+b w+c$

$\mathrm{w}^{(k+1)}=\mathrm{w}^{(k)}-\eta \frac{d E\left(\mathrm{w}^{(k)}\right)}{d \mathrm{w}}$

• Gradient descent with fixed step size $\eta$ to estimate scalar parameter $w$
• Using Taylor expansion

$E(w)=E\left(\mathbf{w}^{(k)}\right)+E^{\prime}\left(\mathbf{w}^{(k)}\right)\left(w-\mathbf{w}^{(k)}\right)+E^{\prime\prime}\left(\mathbf{w}^{(k)}\right)\left(w-\mathbf{w}^{(k)}\right)^2$

• So we can get the optimum step size $\eta_{opt} = E^{\prime\prime}(w^{(k)})^{-1}$
• For $\eta < \eta_{opt}$ the algorithm will converge monotonically
• For $2\eta_{opt} > \eta > \eta_{opt}$, we have oscillating convergence
• For $\eta > 2\eta_{opt}$, we get divergence
• For generic differentiable convex objectives
• also can use Taylor expansion to estimate
• Using Newton's method

$\eta_{o p t}=\left(\frac{d^{2} E\left(\mathrm{w}^{(k)}\right)}{d w^{2}}\right)^{-1}$

### Multivariate inputs

$E=\frac{1}{2} \mathbf{w}^{T} \mathbf{A} \mathbf{w}+\mathbf{w}^{T} \mathbf{b}+c$

• If $A$ is diagonal

$E=\frac{1}{2} \sum_{i}\left(a_{i i} w_{i}^{2}+b_{i} w_{i}\right)+c$

• We can optimize each coordinate independently
• Like $\eta_{1,opt} = a^{-1}_{11}$, $\eta_{2,opt} = a^{-1}_{22}$
• But Optimal learning rate is different for the different coordinates
• If updating gradient descent for entire vector, need to satisfy

$\eta < 2 \min_i \eta_{i,opt}$

• This, however, makes the learning very slow if $\frac{\max_i \eta_{i,opt}}{\min_i\eta_{i,opt}}$ is large
• Solution: Normalize the objective to have identical eccentricity in all directions
• Then all of them will have identical optimal learning rates
• Easier to find a working learning rate
• Target

$E=\frac{1}{2} \widehat{\mathbf{w}}^{T} \widehat{\mathbf{w}}+\hat{\mathbf{b}}^{T} \widehat{\mathbf{w}}+c$

• So let $\widehat{\mathbf{w}}=\mathbf{S} \mathbf{w}$, and $S = A^{0.5}$, $\hat{b} = A^{-0.5}b$ ,$\widehat{\mathbf{w}} = A^{0.5} \mathbf{w}$

$\widehat{\mathbf{w}}^{(k+1)}=\widehat{\mathbf{w}}^{(k)}-\eta \nabla_{\widehat{\mathbf{w}}} E\left(\widehat{\mathbf{w}}^{(k)}\right)^{T}$

$\mathbf{w}^{(k+1)}=\mathbf{w}^{(k)}-\eta \mathbf{A}^{-1} \nabla_{\mathbf{w}} E\left(\mathbf{w}^{(k)}\right)^{T}$

• So we just need to caculate $\mathbf{A}^{-1}$, and the step size of each direction is all the same(1)

• For generic differentiable multivariate convex functions

• Also use Taylor expansion

• $E(\mathbf{w}) \approx E\left(\mathbf{w}^{(k)}\right)+\nabla_{\mathbf{w}} E\left(\mathbf{w}^{(k)}\right)\left(\mathbf{w}-w^{(k)}\right)+\frac{1}{2}\left(\mathbf{w}-w^{(k)}\right)^{T} H_{E}\left(w^{(k)}\right)\left(\mathbf{w}-w^{(k)}\right)+\cdots$

• We get the normalized update rule

• $\mathbf{w}^{(k+1)}=\mathbf{w}^{(k)}-\eta H_{E}\left(\boldsymbol{w}^{(k)}\right)^{-1} \nabla_{\mathbf{w}} E\left(\mathbf{w}^{(k)}\right)^{T}$

• Use quadratic approximations to get the maximum

### Issues

#### Hessian

• For complex models such as neural networks, with a very large number of parameters, the Hessian is extremely difficult to compute
• For non-convex functions, the Hessian may not be positive semi-definite, in which case the algorithm can diverge

#### Learning rate

• For complex models such as neural networks the loss function is often not convex
• $\eta > 2\eta_{opt}$ can actually help escape local optima
• However always having $\eta > 2\eta_{opt}$ will ensure that you never ever actually find a solution
• Using Decaying learning rate