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In calculus and other branches of mathematical analysis, limits involving an algebraic combination of functions in an independent variable may often be evaluated by replacing these functions by their limits; if the expression obtained after this substitution does not provide sufficient information to determine the original limit, then it is said to assume an indeterminate form. More generally, an indeterminate form is a mathematical expression involving 0, 1 and which are not precisely determined, due to their propensity to assume different limits. The term was originally introduced by Cauchy's student Moigno in the middle of the 19th century.
There are seven indeterminate forms which are typically considered in the literature:
The most common example of an indeterminate form occurs as the ratio of two functions, in which both of these functions tend to zero in the limit, and is referred to as "the indeterminate form ". For example, as x approaches 0, the ratios , , and go to , 1, and 0 respectively. In each case, if the limits of the numerator and denominator are substituted, the resulting expression is , which is undefined. In a manner of speaking, can take on the values 0, 1, or , and it is even possible to construct similar examples for which the limit is any particular value.
Not every undefined algebraic expression corresponds to an indeterminate form. For example, the expression 1/0 is undefined as a real number but does not correspond to an indeterminate form, because any limit that gives rise to this form will diverge to infinity.
An indeterminate form expression may have a value in some contexts. For example, if κ is an infinite cardinal number, then expressions 0κ, 00, 1κ and κ0 are well defined in the context of cardinal arithmetic. For more, see Zero to the power of zero. Note that zero to the power infinity is not an indeterminate form.
Some examples and non-examples
Indeterminate form 0/0
As mentioned above,
This is enough to show that is an indeterminate form. Other examples with this indeterminate form include
Direct substitution of the number that approaches into any of these expressions shows that these are examples correspond to the indeterminate form , but these limits can assume many different values. Any desired value can be obtained for this indeterminate form as follows:
The value can also be obtained (in the sense of divergence to infinity):
Indeterminate form 00
The following limits illustrate that the expression 00 is an indeterminate form:
Thus, in general, knowing that and is not sufficient to evaluate the limit
If the functions f and g are analytic at c, and f is positive for x sufficiently close (but not equal) to c, then the limit of will be 1. Otherwise, use the transformation in the table below to evaluate the limit.
Expressions that are not indeterminate forms
The expression is not commonly regarded as an indeterminate form, because there is not an infinite range of values that could approach. Specifically, if f approaches 1 and g approaches 0, then f and g may be chosen so that:
- The limit fails to exist.
In each case the absolute value approaches , and so the quotient must diverge, in the sense of the extended real numbers (in the framework of the projectively extended real line, the limit is the unsigned infinity in all three cases). Similarly, any expression of the form with (including and ) is not an indeterminate form, since a quotient giving rise to such an expression will always diverge.
The expression is not an indeterminate form. The expression has the limiting value 0 for the given individual limits, and the expression is equivalent to .
To see why, let where and By taking the natural logarithm of both sides, we get that which means that
Evaluating indeterminate forms
The adjective indeterminate does not imply that the limit does not exist, as many of the examples above show. In many cases, algebraic elimination, L'Hôpital's rule, or other methods can be used to manipulate the expression so that the limit can be evaluated.
For example, the expression can be simplified to x at any point other than x = 0. Thus, the limit of this expression as x approaches 0 (which depends only on points near 0, not at x = 0 itself) is the limit of x, which is 0. Most of the other examples above can also be evaluated using algebraic simplification.
When two variables and converge to zero at the same point and , they are called equivalent infinitesimal (equiv. ).
Moreover, if variables and are such that and , then:
Here is a brief proof:
Suppose there are two equivalent infinitesimals and .
L'Hôpital's rule is a general method for evaluating the indeterminate forms and . This rule states that (under appropriate conditions)
where f' and g' are the derivatives of f and g. (Note that this rule does not apply to expressions , , and so on, as these expressions are not indeterminate forms.) These derivatives will allow one to perform algebraic simplification and eventually evaluate the limit.
L'Hôpital's rule can also be applied to other indeterminate forms, using first an appropriate algebraic transformation. For example, to evaluate the form 00:
The right-hand side is of the form , so L'Hôpital's rule applies to it. Note that this equation is valid (as long as the right-hand side is defined) because the natural logarithm (ln) is a continuous function; it is irrelevant how well-behaved f and g may (or may not) be as long as f is asymptotically positive.
Although L'Hôpital's rule applies both to and to , one of these forms may be more useful than the other in a particular case (because of the possibility of algebraic simplification afterwards). One can change between these forms, if necessary, by transforming to .
List of indeterminate forms
The following table lists the most common indeterminate forms, and the transformations for applying l'Hôpital's rule.
|Indeterminate form||Conditions||Transformation to 0/0||Transformation to ∞/∞|
- Defined and undefined
- Division by zero
- Extended real number line
- Indeterminate equation
- Indeterminate system
- Indeterminate (variable)
- L'Hôpital's rule
- "The Definitive Glossary of Higher Mathematical Jargon — Indeterminate". Math Vault. 2019-08-01. Retrieved 2019-12-02.
- Weisstein, Eric W. "Indeterminate". mathworld.wolfram.com. Retrieved 2019-12-02.
- "Undefined vs Indeterminate in Mathematics". www.cut-the-knot.org. Retrieved 2019-12-02.
- Louis M. Rotando; Henry Korn (January 1977). "The indeterminate form 00". Mathematics Magazine. 50 (1): 41–42. doi:10.2307/2689754.
- "Table of equivalent infinitesimals" (PDF). Vaxa Software.