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Squeeze theorem

In calculus, the squeeze theorem, (also known as the pinching theorem) is a theorem regarding the limit of a function. This theorem argues that if two functions approach the same limit at a point, and a third function "lies" betwixt those functions; then, the third function also approaches that limit at that point.

If the functions f, g, and h are defined in an interval I containing a except possibly at a itself, and f(x) ≤ g(x) ≤ h(x) for every number x in I for which x ≠ a, and

then .

Example

Consider g(x) = x2 sin 1/x.

Trying to calculate the limit of g as x → 0 is difficult by conventional means; substitution will fail since we have a 1/x in the function. Trying to use L'H˘pital's rule fails too; it does not remove the 1/x term. So we turn to using this result.

Let f(x) = -x2 and h(x) = x2, these constitute lower and upper bounds to g(x) and satisfy then f(x) ≤ g(x) ≤ h(x).

We trivially have (because f and h are polynomials)

We then have

because of (*) and the squeeze theorem.

Proof

It is given that

so by the definition of the
limit of a function at a point, for any ε > 0 there is a δ1 > 0 such that
if 0 < |x - a| < δ1 then |f(x) - L| < ε
if 0 < |x - a| < δ1 then -ε < f(x) - L < ε
if 0 < |x - a| < δ1 then L - ε < f(x) < L + ε
and a δ2 > 0 such that > 0 there is a δ1'' > 0 such that
if 0 < |x - a| < δ2 then |h(x) - L| < ε and
if 0 < |x - a| < δ2 then L - ε < h(x) < L + ε.

Then let δ equal the less of δ1 and δ2 (δ = min(δ1, δ2) ). From the previous statements it follows that
if 0 < |x - a| < δ then L - ε < f(x) and
if 0 < |x - a| < δ then h(x) < L + ε.

It is given that f(x) ≤ g(x) ≤ h(x), so
if 0 < |x - a| < δ then L - ε < f(x) ≤ g(x) ≤ h(x) < L + ε.
if 0 < |x - a| < δ then L - ε < g(x) < L + ε.
if 0 < |x - a| < δ then -ε < g(x) - L < ε.
if 0 < |x - a| < δ then |g(x) - L| < ε.

This fits the definition of a limit for the function g as x approaches a, so .