alternating sum


An alternating sum is a sequenceMathworldPlanetmath of arithmetic operations in which each additionPlanetmathPlanetmath is followed by a subtraction, and viceversa, applied to a sequence of numerical entities. For example,

log2=1-12+13-14+15-16+17-

An alternating sum is also called an alternating series.

Alternating sums are often expressed in summation notation with the iterated expression involving multiplication by negative one raised to the iterator. Since a negative number raised to an odd numberMathworldPlanetmathPlanetmath gives a negative number while raised to an even number gives a positive number (see: factors with minus sign), (-1)i essentially has the effect of turning the odd-indexed terms of the sequence negative but keeping their absolute valuesMathworldPlanetmathPlanetmathPlanetmath the same. Our previous example would thus be restated

log2=i=1(-1)i-11i.

If the operands in an alternating sum decrease in value as the iterator increases, and approach zero, then the alternating sum converges to a specific value. This fact is used in many of the best-known expression for π or fractions thereof, such as the Gregory series:

π4=i=0(-1)i12i+1

Other constants also find expression as alternating sums, such as Cahen’s constant.

An alternating sum need not necessarily involve an infinityMathworldPlanetmathPlanetmath of operands. For example, the alternating factorialMathworldPlanetmath of n is computed by an alternating sum stopping at i=n.

References

  • 1 Tobias Dantzig, Number: The LanguagePlanetmathPlanetmath of Science, ed. Joseph Mazur. New York: Pi Press (2005): 166
Title alternating sum
Canonical name AlternatingSum
Date of creation 2013-03-22 17:35:30
Last modified on 2013-03-22 17:35:30
Owner PrimeFan (13766)
Last modified by PrimeFan (13766)
Numerical id 7
Author PrimeFan (13766)
Entry type Definition
Classification msc 11B25