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The **level-index** (**LI**) representation of numbers, and its algorithms for arithmetic operations, were introduced by Charles Clenshaw and Frank Olver in 1984.^{[1]}

The symmetric form of the LI system and its arithmetic operations were presented by Clenshaw and Peter Turner in 1987.^{[2]}

Michael Anuta, Daniel Lozier, Nicolas Schabanel and Turner developed the algorithm for **symmetric level-index** (**SLI**) arithmetic, and a parallel implementation of it. There has been extensive work on developing the SLI arithmetic algorithms and extending them to complex and vector arithmetic operations.

The idea of the level-index system is to represent a non-negative real number X as

where and the process of exponentiation is performed ℓ times, with . ℓ and f are the **level** and **index** of X respectively. *x* = *ℓ* + *f* is the LI image of X. For example,

so its LI image is

The symmetric form is used to allow negative exponents, if the magnitude of X is less than 1. One takes sgn(log(*X*)) or sgn(|*X*| − |*X*|^{−1}) and stores it (after substituting +1 for 0 for the reciprocal sign since for *X* = 1 = *e*^{0} the LI image is *x* = 1.0 and uniquely defines *X*=1 and we can do away without a third state and use only one bit for the two states −1 and +1^{[clarification needed]}) as the reciprocal sign r_{X}. Mathematically, this is equivalent to taking the reciprocal (multiplicative inverse) of a small magnitude number, and then finding the SLI image for the reciprocal. Using one bit for the reciprocal sign enables the representation of extremely small numbers.

A sign bit may also be used to allow negative numbers. One takes sgn(X) and stores it (after substituting +1 for 0 for the sign since for *X* = 0 the LI image is *x* = 0.0 and uniquely defines *X* = 0 and we can do away without a third state and use only one bit for the two states −1 and +1^{[clarification needed]}) as the sign s_{X}. Mathematically, this is equivalent to taking the inverse (additive inverse) of a negative number, and then finding the SLI image for the inverse. Using one bit for the sign enables the representation of negative numbers.

The mapping function is called the **generalized logarithm function**. It is defined as

and it maps onto itself monotonically and so it is invertible on this interval. The inverse, the **generalized exponential function**, is defined by

The density of values X represented by x has no discontinuities as we go from level ℓ to *ℓ* + 1 (a very desirable property) since:

The generalized logarithm function is closely related to the iterated logarithm used in computer science analysis of algorithms.

Formally, we can define the SLI representation for an arbitrary real X (not 0 or 1) as

where s_{X} is the sign (additive inversion or not) of X and r_{X} is the reciprocal sign (multiplicative inversion or not) as in the following equations:

whereas for X = 0 or 1, we have:

For example,

and its SLI representation is

**^**Clenshaw, Charles William; Olver, Frank William John (1984). "Beyond floating point".*Journal of the ACM*.**31**(2): 319–328. doi:10.1145/62.322429.**^**Clenshaw, Charles William; Turner, Peter R. (1988-10-01) [1986-09-16, 1987-06-04]. "The Symmetric Level-Index System".*IMA Journal of Numerical Analysis*.**8**(4). Oxford University Press, Institute of Mathematics and Its Applications: 517–526. doi:10.1093/imanum/8.4.517. ISSN 0272-4979. OCLC 42026743. Retrieved 2018-07-10.

- Clenshaw, Charles William; Olver, Frank William John; Turner, Peter R. (1989). "Level-index arithmetic: An introductory survey".
*Numerical Analysis and Parallel Processing*(Conference proceedings / The Lancaster Numerical Analysis Summer School 1987). Lecture Notes in Mathematics (LNM).**1397**: 95–168. doi:10.1007/BFb0085718. ISBN 978-3-540-51645-3. - Clenshaw, Charles William; Turner, Peter R. (1989-06-23) [1988-10-04]. "Root Squaring Using Level-Index Arithmetic".
*Computing*.**43**(2). Springer-Verlag: 171–185. doi:10.1007/BF02241860. ISSN 0010-485X. - Zehendner, Eberhard (Summer 2008). "Rechnerarithmetik: Logarithmische Zahlensysteme" (PDF) (Lecture script) (in German). Friedrich-Schiller-Universität Jena. pp. 21–22. Archived (PDF) from the original on 2018-07-09. Retrieved 2018-07-09. [1]
- Hayes, Brian (September–October 2009). "The Higher Arithmetic".
*American Scientist*.**97**(5): 364–368. doi:10.1511/2009.80.364. Archived from the original on 2018-07-09. Retrieved 2018-07-09. [2]. Also reprinted in: Hayes, Brian (2017). "Chapter 8: Higher Arithmetic".*Foolproof, and Other Mathematical Meditations*(1 ed.). The MIT Press. pp. 113–126. ISBN 978-0-26203686-3. ISBN 0-26203686-X.

- sli-c-library (hosted by Google Code), "C++ Implementation of Symmetric Level-Index Arithmetic".