Definition:Decreasing

Ordered Sets
Let $\left({S, \preceq_1}\right)$ and $\left({T, \preceq_2}\right)$ be posets.

Let $\phi: \left({S, \preceq_1}\right) \to \left({T, \preceq_2}\right)$ be a mapping.

Then $\phi$ is decreasing iff:


 * $\forall x, y \in S: x \; \preceq_1 \; y \implies \phi \left({y}\right) \; \preceq_2 \; \phi \left({x}\right)$

Alternative terms are order-inverting, order-reversing, antitone and non-increasing.

Note that this definition also holds if $S = T$.

Real Functions
This definition continues to hold when $S = T = \R$.

Thus, let $f$ be a real function.

Then $f$ is decreasing iff:
 * $x \le y \implies f \left({y}\right) \le f \left({x}\right)$.

Sequences
Let $\left \langle {x_n} \right \rangle$ be a sequence in $\R$.

Then $\left \langle {x_n} \right \rangle$ is decreasing iff:
 * $\forall n \in \N: x_{n+1} \le x_n$

Also see

 * Strictly decreasing
 * Increasing
 * Monotone