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{{Short description|Figurate number}}(File:First six triangular numbers.svg|thumb|The first six triangular numbers (not starting with T0))(File:Triangular Numbers Plot.svg|thumb|Triangular Numbers Plot)A triangular number or triangle number counts objects arranged in an equilateral triangle. Triangular numbers are a type of figurate number, other examples being square numbers and cube numbers. The {{math|n}}th triangular number is the number of dots in the triangular arrangement with {{math|n}} dots on each side, and is equal to the sum of the {{math|n}} natural numbers from 1 to {{math|n}}. The sequence of triangular numbers, starting with the 0th triangular number, is{{block indent|left=1.6|0, 1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78, 91, 105, 120, 136, 153, 171, 190, 210, 231, 253, 276, 300, 325, 351, 378, 406, 435, 465, 496, 528, 561, 595, 630, 666...}}{{OEIS|id=A000217}}

Formula

{{Pascal triangle simplex numbers.svg|2=triangular numbers}}The triangular numbers are given by the following explicit formulas:{{bi|left=1.6|1=displaystylebegin{align} T_n&= sum_{k=1}^n k = 1+2+ dotsb +n & = frac{n^2+n}{2} = frac{n(n+1)}{2} & = {n+1 choose 2}end{align}}}where textstyle {n+1 choose 2} is notation for a binomial coefficient. It represents the number of distinct pairs that can be selected from {{math|n + 1}} objects, and it is read aloud as "{{math|n}} plus one choose two".The fact that the nth triangular number equals n(n+1)/2 can be illustrated using a visual proof.WEB,weblink Triangular Number Sequence, Math Is Fun, For every triangular number T_n, imagine a "half-rectangle" arrangement of objects corresponding to the triangular number, as in the figure below. Copying this arrangement and rotating it to create a rectangular figure doubles the number of objects, producing a rectangle with dimensions n times (n+1), which is also the number of objects in the rectangle. Clearly, the triangular number itself is always exactly half of the number of objects in such a figure, or: T_n = frac{n(n+1)}{2} . The example T_4 follows:{{bi|left=1.6| 2T_4 = 4(4+1) = 20 (green plus yellow) implies that T_4 = frac{4(4+1)}{2} = 10 (green).    (File:Illustration of Triangular Number T 4 Leading to a Rectangle (yellow-green).svg)}}This formula can be proven formally using mathematical induction.BOOK, Spivak, Michael, Michael Spivak, 2008, Calculus, 4th,weblink Houston, Texas, Publish or Perish, 21–22, 978-0-914098-91-1, It is clearly true for 1:T_1 = sum_{k=1}^{1}k = frac{1(1 + 1)}{2} = frac{2}{2} = 1.Now assume that, for some natural number m, T_m = sum_{k=1}^{m}k = frac{m(m + 1)}{2}. Adding m + 1 to this yieldsbegin{align} end{align}so if the formula is true for m, it is true for m+1. Since it is clearly true for 1, it is therefore true for 2, 3, and ultimately all natural numbers n by induction.The German mathematician and scientist, Carl Friedrich Gauss, is said to have found this relationship in his early youth, by multiplying {{math|{{sfrac|n|2}}}} pairs of numbers in the sum by the values of each pair {{math|n + 1}}.WEB, Hayes, Brian, Gauss's Day of Reckoning,weblink American Scientist, Computing Science, 2014-04-16, 2015-04-02,weblink" title="web.archive.org/web/20150402141244weblink">weblink However, regardless of the truth of this story, Gauss was not the first to discover this formula, and some find it likely that its origin goes back to the Pythagoreans in the 5th century BC.WEB, Eves, Howard, Webpage cites AN INTRODUCTION TO THE HISTORY OF MATHEMATICS,weblink Mathcentral, 28 March 2015, The two formulas were described by the Irish monk Dicuil in about 816 in his Computus.Esposito, M. An unpublished astronomical treatise by the Irish monk Dicuil. Proceedings of the Royal Irish Academy, XXXVI C. Dublin, 1907, 378-446. An English translation of Dicuil's account is available.Ross, H.E. & Knott, B.I."Dicuil (9th century) on triangular and square numbers." British Journal for the History of Mathematics, 2019,34 (2), 79-94.weblink{{Anchor|Handshake problem}}The triangular number {{math|T'n}} solves the handshake problem of counting the number of handshakes if each person in a room with {{math|n + 1}} people shakes hands once with each person. In other words, the solution to the handshake problem of {{math|n}} people is {{math|T'n−1}}.WEB,weblink The Handshake Problem | National Association of Math Circles, MathCircles.org, 12 January 2022,weblink" title="web.archive.org/web/20160310182700weblink">weblink 10 March 2016, The function {{math|T}} is the additive analog of the factorial function, which is the products of integers from 1 to {{math|n}}. This same function was coined as the "Termial function"BOOK, Knuth, Donald, The Art of Computer Programming, 3rd, 1, 48, by Donald Knuth's The Art of Computer Programming and denoted n? (analog for the factorial notation n!)For example, 10 termial is equivalent to: which of course, corresponds to the tenth triangular number.The number of line segments between closest pairs of dots in the triangle can be represented in terms of the number of dots or with a recurrence relation:L_n = 3 T_{n-1}= 3{n choose 2};~~~L_n = L_{n-1} + 3(n-1), ~L_1 = 0.In the limit, the ratio between the two numbers, dots and line segments islim_{ntoinfty} frac{T_n}{L_n} = frac{1}{3}.

Relations to other figurate numbers

Triangular numbers have a wide variety of relations to other figurate numbers.Most simply, the sum of two consecutive triangular numbers is a square number, with the sum being the square of the difference between the two (and thus the difference of the two being the square root of the sum). Algebraically,T_n + T_{n-1} = left (frac{n^2}{2} + frac{n}{2}right) + left(frac{left(n-1right)^2}{2} + frac{n-1}{2} right ) = left (frac{n^2}{2} + frac{n}{2}right) + left(frac{n^2}{2} - frac{n}{2} right ) = n^2 = (T_n - T_{n-1})^2.This fact can be demonstrated graphically by positioning the triangles in opposite directions to create a square:{{bi|left=1.6|6 + 10 {{=}} 16     (File:Square number 16 as sum of two triangular numbers.svg)     10 + 15 {{=}} 25     (File:Square number 25 as sum of two triangular numbers.svg)}}The double of a triangular number, as in the visual proof from the above section {{section link||Formula}}, is called a pronic number.There are infinitely many triangular numbers that are also square numbers; e.g., 1, 36, 1225. Some of them can be generated by a simple recursive formula:S_{n+1} = 4S_n left( 8S_n + 1right) with S_1 = 1.All square triangular numbers are found from the recursionS_n = 34S_{n-1} - S_{n-2} + 2 with S_0 = 0 and S_1 = 1.(File:Nicomachus_theorem_3D.svg|thumb|A square whose side length is a triangular number can be partitioned into squares and half-squares whose areas add to cubes. This shows that the square of the {{math|n}}th triangular number is equal to the sum of the first {{math|n}} cube numbers.)Also, the square of the {{math|n}}th triangular number is the same as the sum of the cubes of the integers 1 to {{math|n}}. This can also be expressed as The sum of the first {{mvar|n}} triangular numbers is the {{math|n}}th tetrahedral number: More generally, the difference between the {{math|n}}th {{math|m}}-gonal number and the {{math|n}}th {{math|(m + 1)}}-gonal number is the {{math|(n − 1)}}th triangular number. For example, the sixth heptagonal number (81) minus the sixth hexagonal number (66) equals the fifth triangular number, 15. Every other triangular number is a hexagonal number. Knowing the triangular numbers, one can reckon any centered polygonal number; the {{math|n}}th centered {{math|k}}-gonal number is obtained by the formulaCk_n = kT_{n-1}+1where {{math|T}} is a triangular number.The positive difference of two triangular numbers is a trapezoidal number.The pattern found for triangular numbers sum_{n_1=1}^{n_2}n_1=binom{n_2+1}{2} and for tetrahedral numbers sum_{n_2=1}^{n_3}sum_{n_1=1}^{n_2} n_1=binom{n_3+2}{3}, which uses binomial coefficients, can be generalized. This leads to the formula:JOURNAL, Baumann, Michael Heinrich, 2018-12-12, 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}} File:tetrahedral_triangular_number_10.svg|thumb|The fourth triangular number equals the third tetrahedral number as the nth k-simplex number equals the kth n-simplex number due to the symmetry of Pascal's triangle, and its diagonals being simplex numbers; similarly, the fifth triangular number (15) equals the third pentatope numberpentatope number

Other properties

Triangular numbers correspond to the first-degree case of Faulhaber's formula.Alternating triangular numbers (1, 6, 15, 28, ...) are also hexagonal numbers.Every even perfect number is triangular (as well as hexagonal), given by the formulaM_p 2^{p-1} = frac{M_p (M_p + 1)}2 = T_{M_p}where {{math|Mp}} is a Mersenne prime. No odd perfect numbers are known; hence, all known perfect numbers are triangular.For example, the third triangular number is (3 × 2 =) 6, the seventh is (7 × 4 =) 28, the 31st is (31 × 16 =) 496, and the 127th is (127 × 64 =) 8128.The final digit of a triangular number is 0, 1, 3, 5, 6, or 8, and thus such numbers never end in 2, 4, 7, or 9. A final 3 must be preceded by a 0 or 5; a final 8 must be preceded by a 2 or 7.In base 10, the digital root of a nonzero triangular number is always 1, 3, 6, or 9. Hence, every triangular number is either divisible by three or has a remainder of 1 when divided by 9:{{Block indent|left=1.6|1=1 = 9 × 0 + 13 = 9 × 0 + 36 = 9 × 0 + 610 = 9 × 1 + 115 = 9 × 1 + 621 = 9 × 2 + 328 = 9 × 3 + 136 = 9 × 445 = 9 × 555 = 9 × 6 + 166 = 9 × 7 + 378 = 9 × 8 + 691 = 9 × 10 + 1...}}The digital root pattern for triangular numbers, repeating every nine terms, as shown above, is "1, 3, 6, 1, 6, 3, 1, 9, 9".The converse of the statement above is, however, not always true. For example, the digital root of 12, which is not a triangular number, is 3 and divisible by three.If {{math|x}} is a triangular number, then {{math|ax + b}} is also a triangular number, given {{math|a}} is an odd square and {{math|b {{=}} {{sfrac|a − 1|8}}}}. Note that{{math|b}} will always be a triangular number, because {{math|1=8Tn + 1 = (2n + 1)2}}, which yields all the odd squares are revealed by multiplying a triangular number by 8 and adding 1, and the process for {{math|b}} given {{math|a}} is an odd square is the inverse of this operation.The first several pairs of this form (not counting {{math|1x + 0}}) are: {{math|9x + 1}}, {{math|25x + 3}}, {{math|49x + 6}}, {{math|81x + 10}}, {{math|121x + 15}}, {{math|169x + 21}}, ... etc. Given {{math|x}} is equal to {{math|Tn}}, these formulas yield {{math|T3n + 1}}, {{math|T5n + 2}}, {{math|T7n + 3}}, {{math|T9n + 4}}, and so on.The sum of the reciprocals of all the nonzero triangular numbers is This can be shown by using the basic sum of a telescoping series: Two other formulas regarding triangular numbers areT_{a+b} = T_a + T_b + abandT_{ab} = T_aT_b + T_{a-1}T_{b-1},both of which can easily be established either by looking at dot patterns (see above) or with some simple algebra.In 1796, Gauss discovered that every positive integer is representable as a sum of three triangular numbers (possibly including {{math|T0}} = 0), writing in his diary his famous words, "ΕΥΡΗΚΑ! {{nowrap|1=num = Δ + Δ + Δ}}". This theorem does not imply that the triangular numbers are different (as in the case of 20 = 10 + 10 + 0), nor that a solution with exactly three nonzero triangular numbers must exist. This is a special case of the Fermat polygonal number theorem.The largest triangular number of the form {{math|2k − 1}} is 4095 (see Ramanujan–Nagell equation).WacÅ‚aw Franciszek SierpiÅ„ski posed the question as to the existence of four distinct triangular numbers in geometric progression. It was conjectured by Polish mathematician Kazimierz Szymiczek to be impossible and was later proven by Fang and Chen in 2007.Chen, Fang: Triangular numbers in geometric progressionFang: Nonexistence of a geometric progression that contains four triangular numbersFormulas involving expressing an integer as the sum of triangular numbers are connected to theta functions, in particular the Ramanujan theta function.JOURNAL, Liu, Zhi-Guo, 2003-12-01, An Identity of Ramanujan and the Representation of Integers as Sums of Triangular Numbers, The Ramanujan Journal, en, 7, 4, 407–434, 10.1023/B:RAMA.0000012425.42327.ae, 122221070, 1382-4090, ARXIV, Sun, Zhi-Hong, 2016-01-24, Ramanujan's theta functions and sums of triangular numbers, 1601.06378, math.NT,

Applications

{{central_polygonal_numbers.svg}}A fully connected network of {{math|n}} computing devices requires the presence of {{math|Tn − 1}} cables or other connections; this is equivalent to the handshake problem mentioned above.In a tournament format that uses a round-robin group stage, the number of matches that need to be played between {{math|n}} teams is equal to the triangular number {{math|Tn − 1}}. For example, a group stage with 4 teams requires 6 matches, and a group stage with 8 teams requires 28 matches. This is also equivalent to the handshake problem and fully connected network problems.One way of calculating the depreciation of an asset is the sum-of-years' digits method, which involves finding {{math|T'n}}, where {{math|n}} is the length in years of the asset's useful life. Each year, the item loses {{math|(b − s) × {{sfrac|n − y|T'n}}}}, where {{math|b}} is the item's beginning value (in units of currency), {{math|s}} is its final salvage value, {{math|n}} is the total number of years the item is usable, and {{math|y}} the current year in the depreciation schedule. Under this method, an item with a usable life of {{math|n}} = 4 years would lose {{sfrac|4|10}} of its "losable" value in the first year, {{sfrac|3|10}} in the second, {{sfrac|2|10}} in the third, and {{sfrac|1|10}} in the fourth, accumulating a total depreciation of {{sfrac|10|10}} (the whole) of the losable value.Board game designers Geoffrey Engelstein and Isaac Shalev describe triangular numbers as having achieved "nearly the status of a mantra or koan among game designers", describing them as "deeply intuitive" and "featured in an enormous number of games, [proving] incredibly versatile at providing escalating rewards for larger sets without overly incentivizing specialization to the exclusion of all other strategies".BOOK, Engelstein, Geoffrey, Shalev, Isaac, 2019-06-25, Building Blocks of Tabletop Game Design,weblink 10.1201/9780429430701, 978-0-429-43070-1, 198342061,

Triangular roots and tests for triangular numbers{{Anchor|Triangular root}}

By analogy with the square root of {{mvar|x}}, one can define the (positive) triangular root of {{mvar|x}} as the number {{math|n}} such that {{math|1=Tn = x}}:{{Citation |last1=Euler |first1=Leonhard |author-link=Leonhard Euler |last2=Lagrange |first2=Joseph Louis |author2-link=Joseph Louis Lagrange |year=1810 |title=Elements of Algebra |edition=2nd |volume=1 |publisher=J. Johnson and Co. |pages=332–335|title-link=Elements of Algebra }}n = frac{sqrt{8x+1}-1}{2}which follows immediately from the quadratic formula. So an integer {{mvar|x}} is triangular if and only if {{math|8x + 1}} is a square. Equivalently, if the positive triangular root {{mvar|n}} of {{mvar|x}} is an integer, then {{mvar|x}} is the {{mvar|n}}th triangular number.

Alternative name

As stated, an alternative name proposed by Donald Knuth, by analogy to factorials, is "termial", with the notation {{math|n}}? for the {{math|n}}th triangular number.Donald E. Knuth (1997). The Art of Computer Programming: Volume 1: Fundamental Algorithms. 3rd Ed. Addison Wesley Longman, U.S.A. p. 48. However, although some other sources use this name and notation,{{citation | last = Stone | first = John David | doi = 10.1007/978-3-662-57970-1 | page = 282 | publisher = Springer | title = Algorithms for Functional Programming | year = 2018| isbn = 978-3-662-57968-8 | s2cid = 53079729 }} they are not in wide use.

See also

• 1 + 2 + 3 + 4 + ⋯
• Doubly triangular number, a triangular number whose position in the sequence of triangular numbers is also a triangular number
• Tetractys, an arrangement of ten points in a triangle, important in Pythagoreanism

References

{{Reflist|30em}}

External links

{{Commons category|Triangular numbers|lcfirst=yes}}

• {{SpringerEOM|title=Arithmetic series|id=p/a013370}}
• Triangular numbers at cut-the-knot
• There exist triangular numbers that are also square at cut-the-knot
• {{MathWorld|urlname=TriangularNumber|title=Triangular Number}}
• Hypertetrahedral Polytopic Roots by Rob Hubbard, including the generalisation to triangular cube roots, some higher dimensions, and some approximate formulas

{{Figurate numbers}}{{Series (mathematics)}}{{Classes of natural numbers}}