[OkayPhysicist]

The "squashing function" necessarily is nonlinear in multilayer nueral networks. A single layer of a neural network can be quite simply written a weight matrix, times an input vector, equalling an output vector, like so

Ax = y

Adding another layer is just multiplying a different set of weights times the output of the first, so

B(Ax)= y

If you remember your linear algebra course, you might see the problem: that can be simplified

(BA)x = y

Cx = y

Completely indistinguishable from a single layer, thus only capable of modeling linear relationships.

To prevent this collapse, a non linear function must be introduced between each layer.

[quantadev]

Right. All the squashing is doing is keeping the output of any neuron in a range of below 1.

But the entire NN itself (Perceptron ones, which most LLMs are) is still completely using nothing but linearity to store all the knowledge from the training process. All the weights are just an 'm' in the basic line equation 'y=m*x+b'. The entire training process does nothing but adjust a bunch of slopes of a bunch of lines. It's totally linear. No non-linearity at all.

[nazgul17]

The non linearities are fundamental. Without them, any arbitrarily deep NN is equivalent to a shallow NN (easily computable, as GP was saying), and we know those can't even solve the XOR problem.

> nothing but linearity

No, if you have non linearities, the NN itself is not linear. The non linearities are not there primarily to keep the outputs in a given range, though that's important, too.

[scarmig]

Nonlinearity somewhere is fundamental, but it doesn't need to be between each layer. You can, for instance, project each input to a higher dimensional space with a nonlinearity, and the problem becomes linearly separable with high probability (cf Cover's Theorem).

So, for XOR, (x, y) -> (x, y, xy), and it becomes trivial for a linear NN to solve.

Architectures like Mamba have a linear recurrent state space system as their core, so even though you need a nonlinearity somewhere, it doesn't need to be pervasive. And linear recurrent networks are surprisingly powerful (https://arxiv.org/abs/2303.06349, https://arxiv.org/abs/1802.03308).

[quantadev]

> The non linearities are not there primarily to keep the outputs in a given range

Precisely what the `Activation Function` does is to squash an output into a range (normally below one, like tanh). That's the only non-linearity I'm aware of. What other non-linearities are there?

All the training does is adjust linear weights tho, like I said. All the training is doing is adjusting the slopes of lines.

[uh_uh]

> That's the only non-linearity I'm aware of.

"only" is doing a lot work here because that non-linearity is enough to vastly expand the landscape of functions that an NN can approximate. If the NN was linear, you could greatly simplify the computational needs of the whole thing (as was implied by another commenter above) but you'd also not get a GPT out of it.

[quantadev]

All the trainable parameters are just slopes of lines tho. Training NNs doesn't involve adjusting any inputs to non-linear functions. The tanh smashing function just makes sure nothing can blow up into large numbers and all outputs are in a range of less than 1. There's no "magic" or "knowledge" in the tanh smashing. All the magic is 100% in the weights. They're all linear. The amazing thing is that all weights are linear slopes of lines.

[Nevermark]

Simply squashing the output of a linear signal would be multiplying by a small value. To avoid large y, you add a step y' = y/1000.

That would still be linear. And the result would be that despite squashing, no matter how many layers a model had, it could only fit linear problems. Which can always be fit with a single layer, i.e. single matrix.

So nobody does that.

The nonlinearity doesn't just squash some inputs. But create a new rich feature, decision making. That's because on one side of a threshold y gets converted very differently than another. I.e if y > 0, y' = y, otherwise y = 0.

Now you have a discontinuity in behavior, you have a decision.

Multiple layers making decisions can do far more than a linear layer. They can fit any continuous function (or any function with a finite number of discontinuities) arbitrarily well.

Non-linearities add a fundamental new feature. You can think of that features as being able to make decisions around the non-linear function's decision points.

---

If you need to prove this to yourself with a simple example, try to create an XOR gate with this function:

    y = w1 * x1 + w2 * x2 + b.

Where you can pick w1, w2 and b.

You are welcome to linearly squash the output, i.e. y' = y * w3, for whatever small w3 you like. It won't help.

Layers with non-linear transformations are layers of decision makers.

Layers of linear transforms are just unnecessarily long ways of writing a single linear transform. Even with linear "squashing".

[uh_uh]

> The tanh smashing function just makes sure nothing can blow up into large numbers and all outputs are in a range of less than 1.

That's not the main point even though it probably helps. As OkayPhysicist said above, without a nonlinearity, you could collapse all the weight matrices into a single matrix. If you have 2 layers (same size, for simplicity) described by weight matrices A and B, you could multiply them and get C, which you could use for inference.

Now, you can do this same trick not only with 2 layers but 100 million, all collapsing into a single matrix after multiplication. If the nonlinearities weren't there, the effective information content of the whole NN would collapse into that of a single-layer NN.

[wrs]

With a ReLU activation function, rather than a simple linear function of the inputs, you get a piecewise linear approximation of a nonlinear function.

ReLU enables this by being nonlinear in a simple way, specifically by outputting zero for negative inputs, so each linear unit can then limit its contribution to a portion of the output curve.

(This is a lot easier to see on a whiteboard!)

[quantadev]

ReLU technically has a non-linearity at zero, but in some sense it's still even MORE linear than tanh or sigmoid, so it just demonstrates even better than tanh-type squashing that all this LLM stuff is being done ultimately with straight line math. All a ReLU function does is choose which line to use, a sloped one or a zero one.

[wrs]

Well. The word “linear” the way you use it doesn’t seem to have any particular meaning, certainly not the standard mathematical meaning, so I’m not sure we can make further progress on this explanation.

I’ll just reiterate that the single “technical” (whatever that means) nonlinearity in ReLU is exactly what lets a layer approximate any continuous[*] function.

[*] May have forgotten some more adjectives here needed for full precision.

[jcparkyn]

> squash an output into a range

This isn't the primary purpose of the activation function, and in fact it's not even necessary. For example see ReLU (probably the most common activation function), leaky ReLU, or for a sillier example: https://youtu.be/Ae9EKCyI1xU?si=KgjhMrOsFEVo2yCe

[quantadev]

You can change the subject by bringing up as many different NN architectures, Activation Functions, etc. as you want. I'm telling you the basic NN Perceptron design (what everyone means when they refer to Perceptrons in general), has something like a `tanh` and not only is it's PRIMARY function to squash a number, that's it's ONLY function.

[mr_toad]

You need a non-linear activation function for the universal approximation theorem to hold. Otherwise, as others have said the model just collapses to a single layer.

Technically the output is still what a statistician would call “linear in the parameters”, but due to the universal approximation theorem it can approximate any non-linear function.

https://stats.stackexchange.com/questions/275358/why-is-incr...

[beckhamc]

How was that person derailing the convo? Nothing says an activation function has to "squash" a number to be in some range. Leaky ReLUs for instance do `f(x) = x if x > 0 else ax` (for some coefficient `a != 0`), that doesn't squash `x` to be in any range (unless you want to be peculiar about your precise definition of what it means to squash a number). The function takes a real in `[-inf, inf]` and produces a number in `[-inf, inf]`.

> Sure there's a squashing function on the output to keep it in a range from 0 to 1 but that's done BECAUSE we're just adding up stuff.

It's not because you're "adding up stuff", there is specific mathematical or statistical reason why it is used. For neural networks it's there to stop your multi layer network collapsing to a single layer one (i.e. a linear algebra reason). You can choose whatever function you want, for hidden layers tanh generally isn't used anymore, it's usually some variant of a ReLU. In fact Leaky ReLUs are very commonly used so OP isn't changing the subject.

If you define a "perceptron" (`g(Wx+b)` and `W` is a `Px1` matrix) and train it as a logistic regression model then you want `g` to be sigmoid. Its purpose is to ensure that the output can be interpreted as a probability (given that use the correct statistical loss), which means squashing the number. The inverse isn't true, if I take random numbers from the internet and squash them to `[0,1]` I don't go call them probabilities.

> and not only is it's PRIMARY function to squash a number, that's it's ONLY function.

Squashing the number isn't the reason, it's the side effect. And even then, I just said that not all activation functions squash numbers.

> All the training does is adjust linear weights tho, like I said.

Not sure what your point is. What is a "linear weight"?

We call layers of the form `g(Wx+b)` "linear" layers but that's an abused term, if g() is non-linear then the output is not linear. Who cares if the inner term `Wx + b` is linear? With enough of these layers you can approximate fairly complicated functions. If you're arguing as to whether there is a better fundamental building block then that is another discussion.