Taxicab number
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In mathematics, the nth taxicab number, typically denoted Ta(n) or Taxicab(n), is defined as the smallest integer that can be expressed as a sum of two positive integer cubes in n distinct ways.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The most famous taxicab number is 1729 = Ta(2) = 13 + 123 = 93 + 103, also known as the Hardy–Ramanujan number.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite video</ref>
The name is derived from a conversation Template:Nowrap involving mathematicians G. H. Hardy and Srinivasa Ramanujan. As told by Hardy:
History and definitionEdit
The pairs of summands of the Hardy–Ramanujan number Ta(2) = 1729 were first mentioned by Bernard Frénicle de Bessy, who published his observation in 1657. 1729 was made famous as the first taxicab number in the early 20th century by a story involving Srinivasa Ramanujan in claiming it to be the smallest for his particular example of two summands. In 1938, G. H. Hardy and E. M. Wright proved that such numbers exist for all positive integers n, and their proof is easily converted into a program to generate such numbers. However, the proof makes no claims at all about whether the thus-generated numbers are the smallest possible and so it cannot be used to find the actual value of Ta(n).
The taxicab numbers subsequent to 1729 were found with the help of computers. John Leech obtained Ta(3) in 1957. E. Rosenstiel, J. A. Dardis and C. R. Rosenstiel found Ta(4) in 1989.<ref>Numbers Count column, Personal Computer World, page 234, November 1989</ref> J. A. Dardis found Ta(5) in 1994 and it was confirmed by David W. Wilson in 1999.<ref>Numbers Count column of Personal Computer World, page 610, Feb 1995</ref>Template:Sfn Ta(6) was announced by Uwe Hollerbach on the NMBRTHRY mailing list on March 9, 2008,<ref>NMBRTHRY Archives – March 2008 (#10) "The sixth taxicab number is 24153319581254312065344" by Uwe Hollerbach</ref> following a 2003 paper by Calude et al. that gave a 99% probability that the number was actually Ta(6).Template:Sfn Upper bounds for Ta(7) to Ta(12) were found by Christian Boyer in 2006.<ref>"New Upper Bounds for Taxicab and Cabtaxi Numbers" Christian Boyer, France, 2006–2008</ref>
The restriction of the summands to positive numbers is necessary, because allowing negative numbers allows for more (and smaller) instances of numbers that can be expressed as sums of cubes in n distinct ways. (sequence A293647 in the OEIS). The concept of a cabtaxi number has been introduced to allow for alternative, less restrictive definitions of this nature. In a sense, the specification of two summands and powers of three is also restrictive; a generalized taxicab number allows for these values to be other than two and three, respectively.
Known taxicab numbersEdit
So far, the following 6 taxicab numbers are known:
<math display=block>\begin{align}
\operatorname{Ta}(1) =& \ 2 \\
&= 1^3 + 1^3 \\[6pt]
\operatorname{Ta}(2) =& \ 1729 \\
&= 1^3 + 12^3 \\
&= 9^3 + 10^3 \\[6pt]
\operatorname{Ta}(3) =& \ 87539319 \\
&= 167^3 + 436^3 \\
&= 228^3 + 423^3 \\
&= 255^3 + 414^3 \\[6pt]
\operatorname{Ta}(4) =& \ 6963472309248 \\
&= 2421^3 + 19083^3 \\
&= 5436^3 + 18948^3 \\
&= 10200^3 + 18072^3 \\
&= 13322^3 + 16630^3 \\[6pt]
\operatorname{Ta}(5) =& \ 48988659276962496 \\
&= 38787^3 + 365757^3 \\
&= 107839^3 + 362753^3 \\
&= 205292^3 + 342952^3 \\
&= 221424^3 + 336588^3 \\
&= 231518^3 + 331954^3 \\[6pt]
\operatorname{Ta}(6) =& \ 24153319581254312065344 \\
&= 582162^3 + 28906206^3 \\
&= 3064173^3 + 28894803^3 \\
&= 8519281^3 + 28657487^3 \\
&= 16218068^3 + 27093208^3 \\
&= 17492496^3 + 26590452^3 \\
&= 18289922^3 + 26224366^3
\end{align}</math>
Upper bounds for taxicab numbersEdit
For the following taxicab numbers upper bounds are known:
<math display=block>\begin{align} \operatorname{Ta}(7) \le & \ 24885189317885898975235988544 \\
&=2648660966^3 + 1847282122^3 \\ &= 2685635652^3 + 1766742096^3 \\ &= 2736414008^3 + 1638024868^3 \\ &= 2894406187^3 + 860447381^3 \\ &= 2915734948^3 + 459531128^3 \\ &= 2918375103^3 + 309481473^3\\ &= 2919526806^3 + 58798362^3\\[6pt]
\operatorname{Ta}(8) \le & \ 50974398750539071400590819921724352 \\
&= 299512063576^3 + 288873662876^3 \\
&= 336379942682^3 + 234604829494^3 \\
&= 341075727804^3 + 224376246192^3 \\
&= 347524579016^3 + 208029158236^3 \\
&= 367589585749^3 + 109276817387^3 \\
&= 370298338396^3 + 58360453256^3\\
&= 370633638081^3 + 39304147071^3\\
&= 370779904362^3 + 7467391974^3 \\[6pt]
\operatorname{Ta}(9) \le & \ 136897813798023990395783317207361432493888 \\
&= 41632176837064^3 + 40153439139764^3 \\ &= 46756812032798^3 + 32610071299666^3 \\ &= 47409526164756^3 + 31188298220688^3 \\ &= 48305916483224^3 + 28916052994804^3 \\ &= 51094952419111^3 + 15189477616793^3 \\ &= 51471469037044^3 + 8112103002584^3\\ &= 51518075693259^3 + 5463276442869^3\\ &= 51530042142656^3 + 4076877805588^3\\ &= 51538406706318^3 + 1037967484386^3 \\[6pt]
\operatorname{Ta}(10) \le & \ 7335345315241855602572782233444632535674275447104 \\
&= 15695330667573128^3 + 15137846555691028^3 \\
&= 17627318136364846^3 + 12293996879974082^3 \\
&= 17873391364113012^3 + 11757988429199376^3 \\
&= 18211330514175448^3 + 10901351979041108^3 \\
&= 19262797062004847^3 + 5726433061530961^3 \\
&= 19404743826965588^3 + 3058262831974168^3\\
&= 19422314536358643^3 + 2059655218961613^3\\
&= 19426825887781312^3 + 1536982932706676^3\\
&= 19429379778270560^3 + 904069333568884^3\\
&= 19429979328281886^3 + 391313741613522^3 \\[6pt]
\operatorname{Ta}(11) \le & \ 2818537360434849382734382145310807703728251895897826621632 \\
&= 11410505395325664056^3 + 11005214445987377356^3 \\
&= 12815060285137243042^3 + 8937735731741157614^3 \\
&= 12993955521710159724^3 + 8548057588027946352^3 \\
&= 13239637283805550696^3 + 7925282888762885516^3 \\
&= 13600192974314732786^3 + 6716379921779399326^3 \\
&= 14004053464077523769^3 + 4163116835733008647^3\\
&= 14107248762203982476^3 + 2223357078845220136^3\\
&= 14120022667932733461^3 + 1497369344185092651^3\\
&= 14123302420417013824^3 + 1117386592077753452^3\\
&= 14125159098802697120^3 + 657258405504578668^3\\
&= 14125594971660931122^3 + 284485090153030494^3 \\[6pt]
\operatorname{Ta}(12) \le & \ 73914858746493893996583617733225161086864012865017882136931801625152 \\
&= 33900611529512547910376^3 + 32696492119028498124676^3 \\
&= 38073544107142749077782^3 + 26554012859002979271194^3 \\
&= 38605041855000884540004^3 + 25396279094031028611792^3 \\
&= 39334962370186291117816^3 + 23546015462514532868036^3 \\
&= 40406173326689071107206^3 + 19954364747606595397546^3 \\
&= 41606042841774323117699^3 + 12368620118962768690237^3 \\
&= 41912636072508031936196^3 + 6605593881249149024056^3 \\
&= 41950587346428151112631^3 + 4448684321573910266121^3 \\
&= 41960331491058948071104^3 + 3319755565063005505892^3 \\
&= 41965847682542813143520^3 + 1952714722754103222628^3 \\
&= 41965889731136229476526^3 + 1933097542618122241026^3 \\
&= 41967142660804626363462^3 + 845205202844653597674^3
\end{align}</math>
Cubefree taxicab numbersEdit
A more restrictive taxicab problem requires that the taxicab number be cubefree, which means that it is not divisible by any cube other than 13. When a cubefree taxicab number Template:Mvar is written as Template:Math, the numbers Template:Mvar and Template:Mvar must be relatively prime. Among the taxicab numbers Template:Math listed above, only Template:Math and Template:Math are cubefree taxicab numbers. The smallest cubefree taxicab number with three representations was discovered by Paul Vojta (unpublished) in 1981 while he was a graduate student:
<math display=block>\begin{align}
15170835645 &= 517^3 + 2468^3 \\ &= 709^3 + 2456^3 \\ &= 1733^3 + 2152^3
\end{align}</math>
The smallest cubefree taxicab number with four representations was discovered by Stuart Gascoigne and independently by Duncan Moore in 2003:
<math display=block> \begin{align}
1801049058342701083 &= 92227^3 + 1216500^3 \\ &= 136635^3 + 1216102^3 \\ &= 341995^3 + 1207602^3 \\ &= 600259^3 + 1165884^3
\end{align}</math> (sequence A080642 in the OEIS).
See alsoEdit
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- Template:Annotated link, a list of related conjectures and theorems
NotesEdit
ReferencesEdit
- Template:Cite book
- Template:Cite journal
- Template:Cite journal
- Template:Cite journal (Wilson was unaware of J. A. Dardis' prior discovery of Ta(5) in 1994 when he wrote this.)
- Template:Cite journal
- Template:Cite journal
External linksEdit
- A 2002 post to the Number Theory mailing list by Randall L. Rathbun
- Template:Cite video
- Taxicab and other maths at Euler
- {{#invoke:citation/CS1|citation
|CitationClass=web }}