Squeeze theorem explained

In calculus, the squeeze theorem (also known as the sandwich theorem, among other names) is a theorem regarding the limit of a function that is bounded between two other functions.

The squeeze theorem is used in calculus and mathematical analysis, typically to confirm the limit of a function via comparison with two other functions whose limits are known. It was first used geometrically by the mathematicians Archimedes and Eudoxus in an effort to compute , and was formulated in modern terms by Carl Friedrich Gauss.

Statement

The squeeze theorem is formally stated as follows.^[1]

• 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 .

This theorem is also valid for sequences. Let be two sequences converging to, and a sequence. If

\forall n\geq N, N\in\N we have, then also converges to .

Proof

According to the above hypotheses we have, taking the limit inferior and superior:$$L=\backslash lim\_\; g(x)\backslash leq\backslash liminf\_f(x)\; \backslash leq\; \backslash limsup\_f(x)\backslash leq\; \backslash lim\_h(x)=L,$$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

|x - a| < \delta, we have |f(x) - L| < \varepsilon. Symbolically,

As

$$\backslash lim\_\; g(x)\; =\; L$$

means that

and $$\backslash lim\_\; h(x)\; =\; L$$

means that

then we have

$$g(x)\; \backslash leq\; f(x)\; \backslash leq\; h(x)$$$$g(x)\; -\; L\backslash leq\; f(x)\; -\; L\backslash leq\; h(x)\; -\; L$$

We can choose

\delta: =min \left\{\delta _{1,\delta} _{2\right\}} . Then, if |x - a| < \delta , combining and, we have

which completes the proof. Q.E.D

The proof for sequences is very similar, using the

\varepsilon -definition of the limit of a sequence.

Examples

First example

The limit

$$\backslash lim\_x^2\; \backslash sin\backslash left(\backslash tfrac\; \backslash right)$$

cannot be determined through the limit law

$$\backslash lim\_(f(x)\; \backslash cdot\; g(x))\; =\; \backslash lim\_f(x)\; \backslash cdot\; \backslash lim\_g(x),$$

because

$$\backslash lim\_\backslash sin\backslash left(\backslash tfrac\; \backslash right)$$

does not exist.

However, by the definition of the sine function,

$$-1\; \backslash le\; \backslash sin\backslash left(\backslash tfrac\; \backslash right)\; \backslash le\; 1.$$

It follows that

$$-x^2\; \backslash le\; x^2\; \backslash sin\backslash left(\backslash tfrac\; \backslash right)\; \backslash le\; x^2$$

Since

\lim _{x\to} - x ^{2} = \lim _{x\to} x ^{2} = 0 , by the squeeze theorem, \lim _{x\to} x ^{2} \sin\left(\tfrac{ 1 }{x}\right) must also be 0.

Second example

Probably the best-known examples of finding a limit by squeezing are the proofs of the equalities

The first limit follows by means of the squeeze theorem from the fact that^[2]

$$\backslash cos\; x\; \backslash leq\; \backslash frac\; \backslash leq\; 1$$

for close enough to 0. The correctness of which for positive can be seen by simple geometric reasoning (see drawing) that can be extended to negative as well. The second limit follows from the squeeze theorem and the fact that

$$0\; \backslash leq\; \backslash frac\; \backslash leq\; x$$for close enough to 0. This can be derived by replacing in the earlier fact by $\backslash sqrt$ and squaring the resulting inequality.

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.

Third example

It is possible to show that$$\backslash frac\; \backslash tan\backslash theta\; =\; \backslash sec^2\backslash theta$$by squeezing, as follows.

In the illustration at right, the area of the smaller of the two shaded sectors of the circle is

$$\backslash frac,$$

since the radius is and the arc on the unit circle has length . Similarly, the area of the larger of the two shaded sectors is

$$\backslash frac.$$

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, and the height is 1. The area of the triangle is therefore

$$\backslash frac.$$

From the inequalities

$$\backslash frac\; \backslash le\; \backslash frac\; \backslash le\; \backslash frac$$

we deduce that

$$\backslash sec^2\backslash theta\; \backslash le\; \backslash frac\; \backslash le\; \backslash sec^2(\backslash theta\; +\; \backslash Delta\backslash theta),$$

provided , and the inequalities are reversed if . Since the first and third expressions approach as, and the middle expression approaches

\tfrac{d}{d\theta} \tan\theta, the desired result follows.

Fourth example

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.^[3]

$$\backslash lim\_\; \backslash frac$$

cannot be found by taking any number of limits along paths that pass through the point, but since

therefore, by the squeeze theorem,

$$\backslash lim\_\; \backslash frac\; =\; 0.$$

References

References

1. Book: Sohrab. Houshang H.. Basic Real Analysis. 2003. Birkhäuser. 978-1-4939-1840-9. 104. 2nd.
2. Selim G. Krejn, V.N. Uschakowa: Vorstufe zur höheren Mathematik. Springer, 2013,, pp. 80-81 (German). See also Sal Khan: Proof: limit of (sin x)/x at x=0 (video, Khan Academy)
3. Book: Chapter 15.2 Limits and Continuity. 909–910. Multivariable Calculus. 2008. Stewart. James. James Stewart (mathematician). 6th. 978-0495011637.

External links

• • Squeeze Theorem by Bruce Atwood (Beloit College) after work by, Selwyn Hollis (Armstrong Atlantic State University), the Wolfram Demonstrations Project.
• Squeeze Theorem on ProofWiki.