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{{Short description|Polyhedral number representing a tetrahedron}}
[[Image:Pyramid of 35 spheres animation.gif|frame|right|A pyramid with side length 5 contains 35 spheres. Each layer represents one of the first five triangular numbers.]]
A '''tetrahedral number''', or '''triangular pyramidal number''', is a [[figurate number]] that represents a [[pyramid (geometry)|pyramid]] with a triangular base and three sides, called a [[tetrahedron]]. The {{mvar|n}}th tetrahedral number, {{mvar|Te<sub>n</sub>}}, is the sum of the first {{mvar|n}} [[triangular number]]s, that is,

:<math> Te_n = \sum_{k=1}^n T_k = \sum_{k=1}^n \frac{k(k+1)}{2} = \sum_{k=1}^n \left(\sum_{i=1}^k i\right)</math>

The tetrahedral numbers are:

:[[1]], [[4]], [[10]], [[20 (number)|20]], [[35 (number)|35]], [[56 (number)|56]], [[84 (number)|84]], [[120 (number)|120]], [[165 (number)|165]], [[220 (number)|220]], ... {{OEIS|id=A000292}}

== Formula ==
{{Pascal_triangle_simplex_numbers.svg}}

The formula for the {{mvar|n}}th tetrahedral number is represented by the 3rd [[rising factorial]] of {{mvar|n}} divided by the [[factorial]] of 3:
:<math>Te_n= \sum_{k=1}^n T_k = \sum_{k=1}^n \frac{k(k+1)}{2} = \sum_{k=1}^n \left(\sum_{i=1}^k i\right)=\frac{n(n+1)(n+2)}{6} = \frac{n^{\overline 3}}{3!}</math>

The tetrahedral numbers can also be represented as [[binomial coefficient]]s:
:<math>Te_n=\binom{n+2}{3}.</math>
Tetrahedral numbers can therefore be found in the fourth position either from left or right in [[Pascal's triangle]].

===Proofs of formula===
This proof uses the fact that the {{mvar|n}}th triangular number is given by
:<math>T_n=\frac{n(n+1)}{2}.</math>
It proceeds by [[Mathematical induction|induction]].

;Base case
:<math>Te_1 = 1 = \frac{1\cdot 2\cdot 3}{6}.</math>

;Inductive step
:<math>\begin{align} 
Te_{n+1} \quad 
&= Te_n + T_{n+1} \\
&= \frac{n(n+1)(n+2)}{6} + \frac{(n+1)(n+2)}{2}  \\
&= (n+1)(n+2)\left(\frac{n}{6}+\frac{1}{2}\right) \\
&= \frac{(n+1)(n+2)(n+3)}{6}.
\end{align}</math>

The formula can also be proved by [[Gosper's algorithm]].

===Recursive relation===
Tetrahedral and triangular numbers are related through the recursive formulas

:<math>\begin{align}
& Te_n = Te_{n-1} + T_n &(1)\\
& T_n = T_{n-1} + n &(2)
\end{align}</math>

The equation <math>(1)</math> becomes

:<math>\begin{align}
& Te_n = Te_{n-1} + T_{n-1} + n
\end{align}</math>

Substituting <math>n-1</math> for <math>n</math> in equation <math>(1)</math>

:<math>\begin{align}
& Te_{n-1} = Te_{n-2} + T_{n-1}
\end{align}</math>

Thus, the <math>n</math>th tetrahedral number satisfies the following recursive equation

:<math>\begin{align}
& Te_{n} = 2Te_{n-1} - Te_{n-2} + n
\end{align}</math>

==Generalization==

The pattern found for triangular numbers <math> \sum_{n_1=1}^{n_2}n_1=\binom{n_2+1}{2}</math> and for tetrahedral numbers <math> \sum_{n_2=1}^{n_3}\sum_{n_1=1}^{n_2}n_1=\binom{n_3+2}{3}</math> can be generalized. This leads to the formula:<ref>{{Cite journal|last=Baumann|first=Michael Heinrich|date=2018-12-12|title=Die {{math|''k''}}-dimensionale Champagnerpyramide|journal=Mathematische Semesterberichte|language=de|volume=66|pages=89–100|doi=10.1007/s00591-018-00236-x|s2cid=125426184 |issn=1432-1815|url=https://epub.uni-bayreuth.de/3850/1/Baumann_Champagnerpyramide.pdf }}</ref>
<math display=block> \sum_{n_{k-1}=1}^{n_k}\sum_{n_{k-2}=1}^{n_{k-1}}\ldots\sum_{n_2=1}^{n_3}\sum_{n_1=1}^{n_2}n_1=\binom{n_k+k-1}{k}</math>
== Geometric interpretation ==

Tetrahedral numbers can be modelled by stacking spheres. For example, the fifth tetrahedral number ({{math|''Te''<sub>5</sub> {{=}} 35}}) can be modelled with 35 [[billiard ball]]s and the standard triangular billiards ball frame that holds 15 balls in place. Then 10 more balls are stacked on top of those, then another 6, then another three and one ball at the top completes the tetrahedron.

When order-{{mvar|n}} tetrahedra built from {{math|''Te''<sub>''n''</sub>}} spheres are used as a unit, it can be shown that a space tiling with such units can achieve a densest [[sphere packing]] as long as {{math|''n'' ≤ 4}}.<ref>{{cite web|url=http://www.pisquaredoversix.force9.co.uk/Tetrahedra.htm|title=Tetrahedra|date=21 May 2000|archive-url=https://web.archive.org/web/20000521231622/http://www.pisquaredoversix.force9.co.uk/Tetrahedra.htm|archive-date=2000-05-21}}</ref>{{dubious|reason=diagram does not prove the statement|date=September 2018}}

==Tetrahedral roots and tests for tetrahedral numbers{{Anchor|tetrahedral root}}==
By analogy with the [[cube root]] of {{mvar|x}}, one can define the (real) tetrahedral root of {{mvar|x}} as the number {{math|''n''}} such that {{math|1=''Te''<sub>''n''</sub> = ''x''}}:
<math display=block>n = \sqrt[3]{3x+\sqrt{9{x^2}-\frac{1}{27}}} +\sqrt[3]{3x-\sqrt{9{x^2}-\frac{1}{27}}} -1</math>

which follows  from [[Cardano's formula]]. Equivalently, if the real tetrahedral root {{mvar|n}} of {{mvar|x}} is an integer, {{mvar|x}} is the {{mvar|n}}th tetrahedral number.

== Properties ==
*:{{math|''Te''<sub>''n''</sub> + ''Te''<sub>''n''−1</sub> {{=}} 1<sup>2</sup> + 2<sup>2</sup> + 3<sup>2</sup> ... + ''n''<sup>2</sup>}}, the [[square pyramidal number]]s.
*:{{math|''Te''<sub>''2n+1''</sub> {{=}} 1<sup>2</sup> + 3<sup>2</sup> ... + ''(2n+1)''<sup>2</sup>}}, sum of odd squares.
*:{{math|''Te''<sub>''2n''{{figure space}}{{figure space}}</sub> {{=}} 2<sup>2</sup> + 4<sup>2</sup> ... + ''(2n)''<sup>2</sup>{{figure space}}{{figure space}}}}, sum of even squares.
*[[A. J. Meyl]] proved in 1878 that only three tetrahedral numbers are also [[Square number|perfect squares]], namely:
*:{{math|1=''Te''<sub>1{{figure space}}</sub> = {{figure space}}{{figure space}}1<sup>2</sup> = {{figure space}}{{figure space}}{{figure space}}{{figure space}}1}}
*:{{math|1=''Te''<sub>2{{figure space}}</sub> = {{figure space}}{{figure space}}2<sup>2</sup> = {{figure space}}{{figure space}}{{figure space}}{{figure space}}4}}
*:{{math|1=''Te''<sub>48</sub> = 140<sup>2</sup> = 19600}}.
*[[Sir Frederick Pollock, 1st Baronet|Sir Frederick Pollock]] conjectured that every positive integer is the sum of at most 5 tetrahedral numbers: see [[Pollock tetrahedral numbers conjecture]].
* The only tetrahedral number that is also a [[square pyramidal number]] is 1 (Beukers, 1988), and the only tetrahedral number that is also a [[perfect cube]] is 1.
* The [[infinite sum]] of tetrahedral numbers' reciprocals is {{sfrac|3|2}}, which can be derived using [[telescoping series]]:
*:<math> \sum_{n=1}^{\infty} \frac{6}{n(n+1)(n+2)} = \frac{3}{2}.</math>
* The [[parity (mathematics)|parity]] of tetrahedral numbers follows the repeating pattern odd-even-even-even.
*An observation of tetrahedral numbers:
*:{{math|''Te''<sub>5</sub> {{=}} ''Te''<sub>4</sub> + ''Te''<sub>3</sub> + ''Te''<sub>2</sub> + ''Te''<sub>1</sub>}}
*Numbers that are both triangular and tetrahedral must satisfy the [[binomial coefficient]] equation:
*:<math>T_n=\binom{n+1}{2}=\binom{m+2}{3}=Te_m.</math>
[[File:tetrahedral_triangular_number_10.svg|thumb|The third tetrahedral number equals the fourth triangular number as the ''n''th ''k''-simplex number equals the ''k''th ''n''-simplex number due to the symmetry of [[Pascal's triangle]], and its diagonals being simplex numbers; similarly, the fifth tetrahedral number (35) equals the fourth [[pentatope number]], and so forth]]
: The only numbers that are both tetrahedral and triangular numbers are {{OEIS|id=A027568}}:
:: {{math|1=''Te''<sub>1{{figure space}}</sub> = ''T''<sub>1{{figure space}}{{figure space}}</sub> = {{figure space}}{{figure space}}{{figure space}}1}}
:: {{math|1=''Te''<sub>3{{figure space}}</sub> = ''T''<sub>4{{figure space}}{{figure space}}</sub> = {{figure space}}{{figure space}}10}}
:: {{math|1=''Te''<sub>8{{figure space}}</sub> = ''T''<sub>15{{figure space}}</sub> = {{figure space}}120}}
:: {{math|1=''Te''<sub>20</sub> = ''T''<sub>55{{figure space}}</sub> = 1540}}
:: {{math|1=''Te''<sub>34</sub> = ''T''<sub>119</sub> = 7140}}
*{{math|1=''Te''<sub>''n''</sub>}} is the sum of all products ''p'' × ''q'' where (''p'', ''q'') are ordered pairs and ''p'' + ''q'' = ''n'' + 1
*{{math|1=''Te''<sub>''n''</sub>}} is the number of (''n'' + 2)-bit numbers that contain two runs of 1's in their binary expansion.
* The largest tetrahedral number of the form <math>2^a+3^b+1</math> for some integers <math>a</math> and <math>b</math> is [[8000 (number) | 8436]].

==Popular culture==
[[File:The_Twelve_Days_of_Christmas_visualisation.svg|thumb|Number of gifts of each type and number received each day and their relationship to [[figurate number]]s]]
{{math|''Te''<sub>12</sub> {{=}} 364}} is the total number of gifts "my true love sent to me" during the course of all 12 verses of the carol, "[[The Twelve Days of Christmas (song)|The Twelve Days of Christmas]]".<ref>{{cite news|url=https://mathlesstraveled.com/2006/12/20/the-twelve-days-of-christmas-and-tetrahedral-numbers/|title=The Twelve Days of Christmas and Tetrahedral Numbers|last=Brent|date=2006-12-21|newspaper=The Math Less Traveled|access-date=2017-02-28}}</ref> The cumulative total number of gifts after each verse is also {{math|''Te''<sub>''n''</sub>}} for verse ''n''.

The number of possible [[KeyForge]] three-house combinations is also a tetrahedral number, {{math|''Te''<sub>''n''−2</sub>}} where {{mvar|n}} is the number of houses.

==See also==
* [[Centered triangular number]]

==References==
{{Reflist}}

==External links==
* {{MathWorld |title=Tetrahedral Number |urlname=TetrahedralNumber}}
* [http://demonstrations.wolfram.com/GeometricProofOfTheTetrahedralNumberFormula/ Geometric Proof of the Tetrahedral Number Formula] by Jim Delany, [[The Wolfram Demonstrations Project]].

{{Figurate numbers}}
{{Classes of natural numbers}}

{{DEFAULTSORT:Tetrahedral Number}}
[[Category:Figurate numbers]]
[[Category:Simplex numbers]]
[[Category:Tetrahedra]]