The squeeze theorem is used in calculus and mathematical analysis. It is typically used to confirm the limit of a function via comparison with two other functions whose limits are known or easily computed. It was first used geometrically by the mathematicians Archimedes and Eudoxus in an effort to compute π, and was formulated in modern terms by Gauss.
In many languages (e.g. French, German, Italian and Russian), the squeeze theorem is also known as the two policemen (and a drunk) theorem, or some variation thereof. The story is that if two policemen are escorting a drunk prisoner between them, and both officers go to a cell, then (regardless of the path taken, and the fact that the prisoner may be wobbling about between the policemen) the prisoner must also end up in the cell.
The squeeze theorem is formally stated as follows.
and also suppose that:
- The functions and are said to be lower and upper bounds (respectively) of .
- Here, is not required to lie in the interior of . Indeed, if is an endpoint of , then the above limits are left- or right-hand limits.
- A similar statement holds for infinite intervals: for example, if , then the conclusion holds, taking the limits as .
From the above hypotheses we have, taking the limit inferior and superior:
so all the inequalities are indeed equalities, and the thesis immediately follows.
A direct proof, using the -definition of limit, would be to prove that for all real there exists a real such that for all with , we have . Symbolically,
then we have
We can choose . Then, if , combining (1) and (2), we have
which completes the proof.
cannot be determined through the limit law
does not exist.
However, by the definition of the sine function,
It follows that
Since , by the squeeze theorem, must also be 0.
Probably the best-known examples of finding a limit by squeezing are the proofs of the equalities
The first follows by means of the squeeze theorem from the fact that
for x close enough, but not equal to 0.
These two limits are used in proofs of the fact that the derivative of the sine function is the cosine function. That fact is relied on in other proofs of derivatives of trigonometric functions.
It is possible to show that
by squeezing, as follows.
In the illustration at right, the area of the smaller of the two shaded sectors of the circle is
since the radius is sec θ and the arc on the unit circle has length Δθ. Similarly the area of the larger of the two shaded sectors is
What is squeezed between them is the triangle whose base is the vertical segment whose endpoints are the two dots. The length of the base of the triangle is tan(θ + Δθ) − tan(θ), and the height is 1. The area of the triangle is therefore
From the inequalities
we deduce that
provided Δθ > 0, and the inequalities are reversed if Δθ < 0. Since the first and third expressions approach sec2θ as Δθ → 0, and the middle expression approaches (d/dθ) tan θ, the desired result follows.
The squeeze theorem can still be used in multivariable calculus but the lower (and upper functions) must be below (and above) the target function not just along a path but around the entire neighborhood of the point of interest and it only works if the function really does have a limit there. It can, therefore, be used to prove that a function has a limit at a point, but it can never be used to prove that a function does not have a limit at a point.
cannot be found by taking any number of limits along paths that pass through the point, but since
therefore, by the squeeze theorem,
|This article needs additional citations for verification. (April 2010)|
- Sohrab, Houshang H. (2003). Basic Real Analysis (2nd ed.). New York: Birkhäuser. p. 104. ISBN 978-1-4939-1840-9.
- Stewart, James (2008). "Chapter 15.2 Limits and Continuity". Multivariable Calculus (6th ed.). pp. 909–910. ISBN 0495011630.