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increasing/decreasing/monotone function (Definition)

Definition Let $ A$ be a subset of $ \mathbb{R}$, and let $ f$ be a function from $ f:A\to \mathbb{R}$. Then

  1. $ f$ is increasing or weakly increasing, if $ x\le y$ implies that $ f(x)\le f(y)$ (for all $ x$ and $ y$ in $ A$).
  2. $ f$ is strictly increasing or strongly increasing, if $ x< y$ implies that $ f(x)< f(y)$.
  3. $ f$ is decreasing or weakly decreasing, if $ x\le y$ implies that $ f(x)\ge f(y)$.
  4. $ f$ is strictly decreasing or strongly decreasing if $ x< y$ implies that $ f(x)> f(y)$.
  5. $ f$ is monotone, if $ f$ is either increasing or decreasing.
  6. $ f$ is strictly monotone or strongly monotone, if $ f$ is either strictly increasing or strictly decreasing.

Theorem Let $ X$ be a bounded or unbounded open interval of $ \mathbb{R}$. In other words, let $ X$ be an interval of the form $ X=(a,b)$, where $ a,b\in\mathbb{R}\cup\{-\infty,\infty\}$. Futher, let $ f:X\to \mathbb{R}$ be a monotone function.

  1. The set of points where $ f$ is discontinuous is at most countable [1,2].
  2. [Lebesgue] $ f$ is differentiable almost everywhere ([3], pp. 514).

Bibliography

1
C.D. Aliprantis, O. Burkinshaw, Principles of Real Analysis, 2nd ed., Academic Press, 1990.
2
W. Rudin, Principles of Mathematical Analysis, McGraw-Hill Inc., 1976.
3
F. Jones, Lebesgue Integration on Euclidean Spaces, Jones and Barlett Publishers, 1993.



"increasing/decreasing/monotone function" is owned by Koro. [ full author list (2) | owner history (1) ]
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Also defines:  increasing, decreasing, strictly increasing, strictly decreasing, monotone, monotonic, strictly monotone, strictly monotonic, weakly increasing, weakly decreasing, strongly increasing, strongly decreasing, strongly monotone, weakly monotone, stronly monotonic, weakly monotonic

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example of increasing/decreasing/monotone function (Example) by Johan
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Cross-references: almost everywhere, differentiable, countable, discontinuous, points, monotone function, interval, open interval, unbounded, bounded, implies, function, subset
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This is version 9 of increasing/decreasing/monotone function, born on 2003-04-28, modified 2008-03-27.
Object id is 4228, canonical name is IncreasingdecreasingmonotoneFunction.
Accessed 27566 times total.

Classification:
AMS MSC26A06 (Real functions :: Functions of one variable :: One-variable calculus)
 26A48 (Real functions :: Functions of one variable :: Monotonic functions, generalizations)

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Question on Definition. by hfleitas on 2006-04-15 00:22:03
Let set A = {1,2,3}.

1. How many relations are monotone increasing funtions?
2. How many relations are monotone decreasing funtions?
3. How many relations are strictly increasing funtions?
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