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Sixth power

From Wikipedia, the free encyclopedia
Result of multiplying six instances of a number
64 (26) and 729 (36) cubelets arranged as cubes((22)3 and (32)3, respectively) and as squares((23)2 and (33)2, respectively)

Inarithmetic andalgebra thesixth power of anumbern is the result of multiplying six instances ofn together. So:

n6 =n ×n ×n ×n ×n ×n.

Sixthpowers can be formed by multiplying a number by itsfifth power, multiplying thesquare of a number by itsfourth power, bycubing a square, or by squaring a cube.

The sequence of sixth powers ofintegers are:

0, 1, 64, 729, 4096, 15625, 46656, 117649, 262144, 531441, 1000000, 1771561, 2985984, 4826809, 7529536, 11390625, 16777216, 24137569, 34012224, 47045881, 64000000, 85766121, 113379904, 148035889, 191102976, 244140625, 308915776, 387420489, 481890304, ... (sequenceA001014 in theOEIS)

They include the significantdecimal numbers 106 (amillion), 1006 (ashort-scale trillion and long-scale billion), 10006 (aquintillion and along-scale trillion) and so on.

Squares and cubes

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The sixth powers of integers can be characterized as the numbers that are simultaneously squares and cubes.[1] In this way, they are analogous to two other classes offigurate numbers: thesquare triangular numbers, which are simultaneously square and triangular,and the solutions to thecannonball problem, which are simultaneously square and square-pyramidal.

Because of their connection to squares and cubes, sixth powers play an important role in the study of theMordell curves, which areelliptic curves of the form

y2=x3+k.{\displaystyle y^{2}=x^{3}+k.}

Whenk{\displaystyle k} is divisible by a sixth power, this equation can be reduced by dividing by that power to give a simpler equation of the same form.A well-known result innumber theory,proven byRudolf Fueter andLouis J. Mordell, states that, whenk{\displaystyle k} is an integer that is not divisible by a sixth power (other than the exceptional casesk=1{\displaystyle k=1} andk=432{\displaystyle k=-432}), this equation either has norational solutions with bothx{\displaystyle x} andy{\displaystyle y} nonzero or infinitely many of them.[2]

In thearchaic notation ofRobert Recorde, the sixth power of a number was called the "zenzicube", meaning the square of a cube. Similarly, the notation for sixth powers used in 12th centuryIndian mathematics byBhāskara II also called them either the square of a cube or the cube of a square.[3]

Sums

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There are numerous known examples of sixth powers that can be expressed as the sum of seven other sixth powers, but no examples are yet known of a sixth power expressible as the sum of just six sixth powers.[4] This makes it unique among the powers with exponentk = 1, 2, ... , 8, the others of which can each be expressed as the sum ofk otherk-th powers, and some of which (in violation ofEuler's sum of powers conjecture) can be expressed as a sum of even fewerk-th powers.

In connection withWaring's problem, every sufficiently large integer can be represented as a sum of at most 24 sixth powers of integers.[5]

There are infinitely many different nontrivial solutions to theDiophantine equation[6]

a6+b6+c6=d6+e6+f6.{\displaystyle a^{6}+b^{6}+c^{6}=d^{6}+e^{6}+f^{6}.}

It has not been proven whether the equation

a6+b6=c6+d6{\displaystyle a^{6}+b^{6}=c^{6}+d^{6}}

has a nontrivial solution,[7] but theLander, Parkin, and Selfridge conjecture would imply that it does not.

Other properties

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See also

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References

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  1. ^Dowden, Richard (April 30, 1825),"(untitled)",Mechanics' Magazine and Journal of Science, Arts, and Manufactures, vol. 4, no. 88, Knight and Lacey, p. 54
  2. ^Ireland, Kenneth F.; Rosen, Michael I. (1982),A classical introduction to modern number theory, Graduate Texts in Mathematics, vol. 84, Springer-Verlag, New York-Berlin, p. 289,ISBN 0-387-90625-8,MR 0661047.
  3. ^Cajori, Florian (2013),A History of Mathematical Notations, Dover Books on Mathematics, Courier Corporation, p. 80,ISBN 9780486161167
  4. ^Quoted inMeyrignac, Jean-Charles (14 February 2001)."Computing Minimal Equal Sums Of Like Powers: Best Known Solutions". Retrieved17 July 2017.
  5. ^Vaughan, R. C.; Wooley, T. D. (1994), "Further improvements in Waring's problem. II. Sixth powers",Duke Mathematical Journal,76 (3):683–710,doi:10.1215/S0012-7094-94-07626-6,MR 1309326
  6. ^Brudno, Simcha (1976), "Triples of sixth powers with equal sums",Mathematics of Computation,30 (135):646–648,doi:10.1090/s0025-5718-1976-0406923-6,MR 0406923
  7. ^Bremner, Andrew; Guy, Richard K. (1988), "Unsolved Problems: A Dozen Difficult Diophantine Dilemmas",American Mathematical Monthly,95 (1):31–36,doi:10.2307/2323442,JSTOR 2323442,MR 1541235

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