# Introduction¶

You must unlearn what you have learned.

—Yoda, The Empire Strikes Back

Our first step is to discretize the real numbers—specifically, to replace them with a finite surrogate set of numbers. This step keeps the time and storage requirements for operating with each number at constant levels, but (almost) every data set and arithmetic operation is perturbed slightly away from its idealized mathematical value. We can easily keep the individual roundoff errors very small, so small that simple random accumulation is unlikely to bother us. However, some problems are extremely sensitive to these perturbations, a trait we quantify using a condition number. Problems with large condition numbers are difficult to solve accurately. Furthermore, even when the condition number of a problem is not large, some algorithms for solving it allow errors to grow enormously. We call these algorithms unstable. In this chapter we discuss these ideas in simple settings before moving on to the more realistic problems in the rest of the book.

## Important terms¶

algorithm

Set of instructions for transforming data into a result.

backward error

Change in the input required to produce the result found by an inexact algorithm.

condition number

Ratio of the size of change in the output of a function to the size of change in the infinitesimal input that produced it.

double precision

Typical standard in floating-point representation, using 64 bits to achieve about 16 decimal significant digits of precision.

floating point numbers

A finite set that substitutes for the real numbers in machine calculations. Denoted by $$\mathbb{F}$$.

ill-conditioned

Exhibiting a large condition number, indicating high sensitivity of a result to changes in the data.

machine epsilon

Distance from 1 to the next-largest floating point number. Also called unit roundoff or machine precision, though the usages are not completely consistent across different references.

subtractive cancellation

Growth in relative error that occurs when two numbers are added/subtracted to get a result that is much smaller in magnitude than the operands.

unstable

Allowing perturbations of the data to have much larger effects on the results than can be explained by the problem’s condition number.

## Important Julia commands and keywords¶

function

Keyword used to start a function definition.

NaN

Not a Number, the result of an undefined arithmetic operation.