Division by infinity

Short description: Mathematical problem

The hyperbola y=1/x. As x approaches ∞, y approaches 0.

In mathematics, division by infinity is division where the divisor (denominator) is infinity. In ordinary arithmetic, this does not have a well-defined meaning, since ∞ is a mathematical concept that does not correspond to a specific number, and moreover, there is no nonzero real number that, when added to itself an infinite number of times, gives a finite number, unless you address the concept of indeterminate forms. However, "dividing by ∞" can be given meaning as an informal way of expressing the limit of dividing a number by larger and larger divisors.[1]: 201–204

Using mathematical structures that go beyond the real numbers, it is possible to define numbers that have infinite magnitude yet can still be manipulated in ways much like ordinary arithmetic. For example, on the extended real number line, dividing any real number by infinity yields zero,[2] while in the surreal number system, dividing 1 by the infinite number ω yields the infinitesimal number ϵ.[3][4]: 12 In floating-point arithmetic, any finite number divided by ±∞ is equal to positive or negative zero if the numerator is finite. Otherwise, the result is NaN.

The challenges of providing a rigorous meaning of "division by infinity" are analogous to those of defining division by zero.

## Contents

* 1 Use in technology
* 2 Use in calculus
* 2.1 Integration
* 2.2 L'Hôpital's rule
* 3 References
* 4 External links

## Use in technology

This calculator does not have an infinity button.

As infinity is difficult to deal with for most calculators and computers, many do not have a formal way of computing division by infinity.[5][6] Calculators such as the TI-84 and most household calculators do not have an infinity button so it is impossible to type into the calculator 'x divided by infinity' so instead users can type a large number such as "1e99" (1×1099) or "-1e99". By typing in some number divided by a sufficiently large number the output will be 0. In some cases this fails as there is either an overflow error or if the numerator is also a sufficiently large number then the output may be 1 or a real number. In the Wolfram language, dividing an integer by infinity will result in the result 0.[7] Also, in some calculators such as the TI-Nspire, 1 divided by infinity can be evaluated as 0.

## Use in calculus

### Integration

Main article: Integral calculus

In calculus, taking the integral of a function is defined finding the area under a curve. This can be done simply by breaking up this area into rectangular sections and taking the sum of these sections. These are called Riemann sums. As the sections get narrower, the Riemann sum becomes an increasingly accurate approximation of the true area. Taking the limit of these Riemann sums, in which the sections can be heuristically regarded as "infinitely thin", gives the definite integral of the function over the prescribed interval. Conceptually this results in dividing the interval by infinity to result in infinitely small pieces.[1]: 255–259

On a different note when taking an integral where one of the boundaries is infinity this is defined as an improper integral.[8] To determine this one would take the limit as a variable a approaches infinity substituting a in for the infinity sign. This would then allow the integral to be evaluated and then the limit would be taken. In many cases evaluating this would result in a term being divided by infinity. In this case in order to evaluate the integral one would assume this to be zero. This allows for the integral to be assumed to converge meaning a finite answer can be determined from the integral using this assumption.[8]

### L'Hôpital's rule

Main articles: Indeterminate form and L'Hôpital's rule

When given a ratio between two functions, the limit of this ratio can be evaluated by computing the limit of each function separately. Where the limit of the function in the denominator is infinity, and the numerator does not allow the ratio to be well determined, the limit of the ratio is said to be of indeterminate form.[9] An example of this is:

Using L'Hôpital's rule to evaluate limits of fractions where the denominator tends towards infinity can produce results other than 0.

limx->cf′(x)g′(x)

then

limx->cf(x)g(x)=limx->cf′(x)g′(x)

So if

limx->c|f(x)|=limx->c|g(x)|=∞,

then

limx->cf(x)g(x)=L[10]

This means that, when using limits to give meaning to division by infinity, the result of "dividing by infinity" does not always equal 0.

## References

1. ↑ 1.0 1.1 Cheng, Eugenia (2018). Beyond Infinity: An Expedition to the Outer Limits of Mathematics. Basic Books. ISBN 9781541644137. OCLC 1003309980.
2. ↑ Hansen, Eldon R.; Walster, G. William (2004). Global Optimization using Interval Analysis (2nd ed.). New York: Marcel Dekker. pp. 57. ISBN 0-8247-5870-6. OCLC 55013079.
3. ↑ Knuth, Donald Ervin (1974). Surreal Numbers: How two ex-students turned on to pure mathematics and found total happiness. Boston, Mass.: Addison-Wesley. pp. 109. ISBN 978-0-201-03812-5. OCLC 1194979.
4. ↑ Conway, John H. (2001). On Numbers and Games (2nd ed.). A K Peters. ISBN 1-56881-127-6.
5. ↑ Zhang, Yin (1998). "Solving large-scale linear programs by interior-point methods under the Matlab ∗ Environment †" (in en). Optimization Methods and Software 10 (1): 1–31. doi:10.1080/10556789808805699. ISSN 1055-6788. http://www.tandfonline.com/doi/abs/10.1080/10556789808805699.
6. ↑ Maniatakos, M.; Kudva, P.; Fleischer, B. M.; Makris, Y. (2013). "Low-Cost Concurrent Error Detection for Floating-Point Unit (FPU) Controllers". IEEE Transactions on Computers 62 (7): 1376–1388. doi:10.1109/TC.2012.81. ISSN 0018-9340.
7. ↑ "Wolfram|Alpha: Making the world's knowledge computable" (in en-us). http://www.wolframalpha.com/input/?i=x/infinity.
8. ↑ 8.0 8.1 (in en) Introduction to improper integrals, https://www.khanacademy.org/math/ap-calculus-bc/bc-integration-new/bc-6-13/v/introduction-to-improper-integrals, retrieved 2018-10-30
9. ↑ Menz, Petra; Mulberry, Nicola (July 13, 2020). "Indeterminate Form & L'Hôpital's Rule". Simon Fraser University. https://www.sfu.ca/math-coursenotes/Math 157 Course Notes/sec_Hopital.html.
10. ↑ "IMOmath: L'Hopital's Theorem". https://www.imomath.com/index.php?options=686.

## External links

* How to Divide by Zero by Bill Shillito

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