Appendix A. Mathematics Terminology Reference

To serve as either a refresher or an introduction, Table A-1 summarizes the key mathematical terminology in the context in which it is used in this book.

Table A-1. Mathematics terminology summary
Terminology Description
x A nonbold variable refers to a scalar.
x A variable in bold refers to a vector.
y = f(x; Θ) The result of function f on the vector input x, where f is dependent on the parameters Θ. In the context of this book, this represents the output of a DNN model for a particular input: f represents the DNN model algorithm, Θ represents its parameters determined during training, and x is the input to the model.
C(f(x; Θ), y) The result of function C given f(x; Θ) and the vector y. In the context of this book, this represents the cost (or loss) of the DNN model for a particular input with respect to the required output y.
xi The element i of vector x.
dydx The derivative of y with respect to x.
yx The partial derivative of y with respect to x, wh - ere x is one of the variables that affects y.
xf The nabla (upside-down Greek delta) symbol means “gradient.” xf refers to the vector of partial derivatives of the function f for the vector x. Put more simply, this means the effect that a very small change to the value of x has on the function f.
{1, 2, . . . L} The set of numbers from 1 to L.
The set of real numbers.
{x : P(x)} The set of all values of x for which P(x) is true, where P(x) is a boolean statement.
argminx{f(x):P(x)} Returns x at which f(x) is minimized such that P(x) is true.
x ∈ ℝ Indicates that x belongs to the set of real numbers.
||x||p The Lp-norm of the vector x. Lp-norms are explained in “A Mathematical Approach to Measuring Perturbation”.
x The sum of all the possible values of x.
xtx The sum of all the possible values of x where xt.
ε The Greek letter ε (epsilon) is used to indicate an extremely small (infinitesimal) quantity.
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