# Equivalence of Definitions of Ceiling Function

## Theorem

Let $x$ be a real number.

The following definitions of the concept of **Ceiling Function** are equivalent:

### Definition 1

The **ceiling function of $x$** is defined as the infimum of the set of integers no smaller than $x$:

- $\ceiling x := \inf \set {m \in \Z: x \le m}$

where $\le$ is the usual ordering on the real numbers.

### Definition 2

The **ceiling function of $x$**, denoted $\ceiling x$, is defined as the smallest element of the set of integers:

- $\set {m \in \Z: x \le m}$

where $\le$ is the usual ordering on the real numbers.

### Definition 3

The **ceiling function of $x$** is the unique integer $\ceiling x$ such that:

- $\ceiling x - 1 < x \le \ceiling x$

## Proof

### Definition 1 equals Definition 2

Follows from Infimum of Set of Integers equals Smallest Element.

$\Box$

### Definition 1 equals Definition 3

Let $S$ be the set:

- $S = \left\{ {m \in \Z: m \ge x}\right\}$

Let $n = \inf S$.

By Infimum of Set of Integers is Integer, $n \in \Z$.

By Infimum of Set of Integers equals Smallest Element, $n\in S$.

Because $n\in S$, we have $n \geq x$.

Because $n-1 < n$, we have by definition of supremum:

- $n-1 \notin S$

Thus $n-1 < x$.

Thus $n$ is an integer such that:

- $n-1 < x \leq n$

So $n$ is the ceiling function by definition 3.

$\Box$

### Definition 3 equals Definition 2

Let $n$ be an integer such that:

- $n-1 < x \leq n$

We show that $n$ is the smallest element of the set:

- $S = \left\{ {m \in \Z: m \ge x}\right\}$

Let $m \in \Z$ such that $m \ge x$.

We show that $n\leq m$.

Aiming for a contradiction, suppose $m < n$.

By Weak Inequality of Integers iff Strict Inequality with Integer minus One:

- $m \le n - 1$

and so from the definition of $g$ it follows that $m < x$.

By Proof by Contradiction it follows that $m \ge n$.

Because $m \in S$ was arbitrary, $n$ is the smallest element of $S$.

Thus $n$ is the ceiling function by definition 2.

$\blacksquare$