Definition:Strictly Decreasing

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Ordered Sets

Let $\left({S, \preceq_1}\right)$ and $\left({T, \preceq_2}\right)$ be ordered sets.

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

Then $\phi$ is strictly decreasing if and only if:

$\forall x, y \in S: x \prec_1 y \implies \phi \left({y}\right) \prec_2 \phi \left({x}\right)$

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

Real Functions

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

Let $f$ be a real function.

Then $f$ is strictly decreasing iff:

$x < y \implies f \left({y}\right) < f \left({x}\right)$


Let $\left({S, \preceq}\right)$ be a totally ordered set.

Then a sequence $\left \langle {a_k} \right \rangle_{k \in A}$ of terms of $S$ is strictly decreasing iff:

$\forall j, k \in A: j < k \implies a_k \prec a_j$

Also see