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Partial derivative

As explained in Math and Architectures of Deep Learning

The derivative of a multivariable function with respect to one variable while holding the others constant, here squared and accumulated by AdaGrad.

A partial derivative measures how a multivariable function changes when exactly one variable changes while all others remain constant. For the loss function L(w0, w1) = 2w0² + 3w1², the partial derivatives isolate each parameter’s contribution to loss change. Combining these contributions estimates the total change when several parameters move together. section 3.3.1

A quadratic loss surface

At P = (w0, w1) = (3, 4), the loss is 66 and the gradient is [12, 24]ᵀ. Moving in the opposite direction, [-12, -24]ᵀ, produces the greatest local reduction in loss. This illustrates how individual partial derivatives combine into a direction for changing parameters. section 3.3.1

Figure 3.4 δL in terms of δw in one dimension, illustrated with two example curves: a straight line and a parabola. In general, δL = (dL/dw) δw. To decrease loss, δw must have the opposite sign of the derivative dL/dw. In (a), this implies we always have to move left (decrease w) to decrease L. In (b), if we are in the left half (e.g., point Q), the derivative is negative, and we have to move to the right to decrease L. But if we are in the right half, the derivative is positive, and we have to move to the left to decrease L. Geometrically, this is equivalent to following the tangent “downward.”

(a) Line: L(w) = 2w + 1, dL/dw = m

(b) Parabola: L(w) = w2, dL/dw = 2w

Use the line-and-parabola comparison to connect one-variable derivative direction with the multivariable idea of changing parameters to decrease loss. section 3.3.1

How Math and Architectures of Deep Learning covers it

How deep it goes
Defines coordinate-wise change, gives a quadratic example, and connects partial derivatives to total change and gradient direction.
Examples
Uses L(w0, w1) = 2w0² + 3w1², point P = (3, 4), and line-and-parabola geometry.
What you can do after
Evaluate how parameter-wise changes combine and identify the direction that locally decreases loss most.
How it connects
Connects partial derivatives to gradients, loss functions, displacement vectors, and total change.

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