Calculus How To

Monotonic Sequence, Series (Monotone): Definition

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Contents:

  1. Monotonic Sequences and Series
  2. Monotonic Functions

What are Monotonic Sequences and Series?

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A monotonic (monotone) sequence or monotone series, is always either steadily increasing or steadily decreasing.

More formally, a series {an} is monotonic if either:

  • ai + 1 ≥ 1 for every i ≥ 1
  • ai + 1 ≤ 1 for every i ≥ 1

If the first is true, the series is monotonically increasing.
If the second is true, it is monotonically decreasing.

Monotonic Sequence: Examples

One example of a monotonically increasing series is the series where an equals

monotonic series

We can tell that this series is steadily increasing if we let an be represented by a function f(n). Then we will want to take that function’s derivative, and get

monotonic sequence derivative


The square in this equation means that it is positive everywhere. This tells us the function is an increasing function, so as n increases an will always increase.

Cautions About Identifying Monotonic Sequences

It’s very important not to just evaluate a few terms and make judgments on whether a sequence is monotonic based on those evaluations. Some sequences seem to increase or decrease steadily for a definite amount of terms, and then suddenly change directions. A sequence may increase for half a million terms, then decrease; such a sequence is not monotonic.

If {an} is both a bounded sequence and a monotonic sequence, we know it is convergent. If {an} is convergent, though, it may or may not be monotonic. There are some convergent series which change direction frequently as they approach a point in an oscillating manner from different sides.

Monotonic Functions

Monotonic functions are always headed in one direction: up or down, never reversing direction.

  • A monotonically (strictly) increasing function (also called strictly increasing) is always headed up; As x increases in the positive direction, so does f(x).
  • A monotonically decreasing function (also called strictly decreasing) is always headed down; As x increases in the positive direction, f(x) decreases.


Determining if a Function is Monotonically Increasing or Decreasing

A monotonically increasing function has a positive derivative (slope) for all points. The reverse is true for monotonically decreasing functions; The derivative is negative at all points. Therefore, if you can figure out what the derivative is doing, you can classify a function as either monotonically increasing or monotonically decreasing.


The above graph shows the function y = 5 + 2x. The graph gives us a visual confirmation that the function is most probably monotonic increasing . However, we can’t see any end behavior on this particular graph, so we can’t say for sure that a function is monotonic just by eyeballing a graph. A sure way is to look at the derivative. For this particular function, the derivative (using the power rule) is
y’ = x2
The function is monotonically increasing because the derivative is positive at all points in the domain. That is, because:

Strictly Monotonic

A strictly monotonic function either increases or decreases steadily as x increases from point a to point b. The difference between strictly monotonic and plain old “monotonic” is that a monotonic function can have areas where the graph flattens out (i.e. where the derivative is zero). A strictly monotonic function cannot have parts of the function that remain constant; the graph must always be increasing, or decreasing, for every single point in the domain.

Monotonic Functions and Discontinuities

Although a monotonic function can have areas where the graph stays constant, there are no discontinuities of the second kind and the function is continuous at “almost all” points across its domain (Houshang, 2003).

References

Dadkhah, K. (2011). Foundations of Mathematical and Computational Economics. Springer Science and Business Media.
Jeffrey, A. Mathematics for Engineers and Scientists, 5th Edition. Mathematics for Engineers and Scientists, 5th Edition. CRC Press.
Houshang, S. (2003). Basic Real Analysis. Springer Science and Business Media.
O’Connor, John. Monotonic Sequences. A First Analysis Course. Math Lecture Notes. University of St. Andrews, Scotland. http://www-history.mcs.st-and.ac.uk/~john/analysis/Lectures/L8.html on June 27, 2019


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Stephanie Glen. "Monotonic Sequence, Series (Monotone): Definition" From CalculusHowTo.com: Calculus for the rest of us! https://www.calculushowto.com/sequence-and-series/monotonic-sequence-series-function/
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