Heronian triangle
In geometry, a Heronian triangle is a triangle that has side lengths and area that are all integers.^{[1]}^{[2]} Heronian triangles are named after Hero of Alexandria. The term is sometimes applied more widely to triangles whose sides and area are all rational numbers,^{[3]} since one can rescale the sides by a common multiple to obtain a triangle that is Heronian in the above sense.
Contents
Properties
Any rightangled triangle whose sidelengths are a Pythagorean triple is a Heronian triangle, as the side lengths of such a triangle are integers, and its area is also an integer, being half of the product of the two shorter sides of the triangle, at least one of which must be even.
An example of a Heronian triangle which is not rightangled is the isosceles triangle with sidelengths 5, 5, and 6, whose area is 12. This triangle is obtained by joining two copies of the rightangled triangle with sides 3, 4, and 5 along the sides of length 4. This approach works in general, as illustrated in the adjacent picture. One takes a Pythagorean triple (a, b, c), with c being largest, then another one (a, d, e), with e being largest, constructs the triangles with these sidelengths, and joins them together along the sides of length a, to obtain a triangle with integer side lengths c, e, and b + d, and with area
 (one half times the base times the height).
If a is even then the area A is an integer. Less obviously, if a is odd then A is still an integer, as b and d must both be even, making b+d even too.
Some Heronian triangles cannot be obtained by joining together two rightangled triangles with integer sides as described above. For example, a 5, 29, 30 Heronian triangle with area 72 cannot be constructed from two integer Pythagorean triangles since none of its altitudes are integers. Also no primitive Pythagorean triangle can be constructed from two smaller integer Pythagorean triangles.^{[4]}^{:p.17} Such Heronian triangles are known as indecomposable.^{[4]} However, if one allows Pythagorean triples with rational values, not necessarily integers, then a decomposition into right triangles with rational sides always exists,^{[5]} because every altitude of a Heronian triangle is rational (since it equals twice the integer area divided by the integer base). So the Heronian triangle with sides 5, 29, 30 can be constructed from rational Pythagorean triangles with sides 7/5, 24/5, 5 and 143/5, 24/5, 29. Note that a Pythagorean triple with rational values is just a scaled version of a triple with integer values.
Other properties of Heronian triangles are as follows:
 The perimeter of a Heronian triangle is always an even number.^{[6]} Thus every Heronian triangle has an odd number of sides of even length,^{[7]}^{:p.3} and every primitive Heronian triangle has exactly one even side.
 The semiperimeter s of a Heronian triangle with sides a, b and c can never be prime. This can be seen from the fact that s(s−a)(s−b)(s−c) has to be a perfect square and if s is a prime then one of the other terms must have s as a factor but this is impossible as these terms are all less than s.
 The area of a Heronian triangle is always divisible by 6.^{[6]}
 All the altitudes of a Heronian triangle are rational.^{[8]} This can be seen from the fact that the area of a triangle is half of one side times its altitude from that side, and a Heronian triangle has integer sides and area. Some Heronian triangles have three noninteger altitudes, for example the acute (15, 34, 35) with area 252 and the obtuse (5, 29, 30) with area 72. Any Heronian triangle with one or more noninteger altitudes can be scaled up by a factor equalling the least common multiple of the altitudes' denominators in order to obtain a similar Heronian triangle with three integer altitudes.
 Heronian triangles that have no integer altitude (indecomposable and nonPythagorean) have sides that are all divisible by primes of the form 4k+1.^{[4]} However decomposable Heronian triangles must have two sides that are the hypotenuse of Pythagorean triangles. Hence all Heronian triangles that are not Pythagorean have at least two sides that are divisible by primes of the form 4k+1. All that remains are Pythagorean triangles. Therefore all Heronian triangles have at least one side that is divisible by primes of the form 4k+1. Finally if a Heronian triangle has only one side divisible by primes of the form 4k+1 it has to be Pythagorean with the side as the hypotenuse and the hypotenuse must be divisible by 5.
 All the interior perpendicular bisectors of a Heronian triangle are rational: For any triangle these are given by and where the sides are a ≥ b ≥ c and the area is A;^{[9]} in a Heronian triangle all of a, b, c, and A are integers.
 There are no equilateral Heronian triangles.^{[8]}
 There are no Heronian triangles with a side length of either 1 or 2.^{[10]}
 There exist an infinite number of primitive Heronian triangles with one side length equal to a provided that a > 2.^{[10]}
 There are no Heronian triangles whose side lengths form a geometric progression.^{[11]}
 If any two sides (but not three) of a Heronian triangle have a common factor, that factor must be the sum of two squares.^{[12]}
 Every angle of a Heronian triangle has a rational sine. This follows from the area formula Area = (1/2)ab sin C, in which the area and the sides a and b are integers (and equivalently for the other angles). Since all integer triangles have all angles' cosines rational, this implies that each oblique angle of a Heron triangle has a rational tangent.
 There are no Heronian triangles whose three internal angles form an arithmetic progression. This is because all plane triangles with angles in an arithmetic progression must have one angle of 60°, which does not have a rational sine.^{[13]}
 Any square inscribed in a Heronian triangle has rational sides: For a general triangle the inscribed square on side of length a has length where A is the triangle's area;^{[14]} in a Heronian triangle, both A and a are integers.
 Every Heronian triangle has a rational inradius (radius of its inscribed circle): For a general triangle the inradius is the ratio of the area to half the perimeter, and both of these are rational in a Heronian triangle.
 Every Heronian triangle has a rational circumradius (the radius of its circumscribed circle): For a general triangle the circumradius equals onefourth the product of the sides divided by the area; in a Heronian triangle the sides and area are integers.
 In a Heronian triangle the distance from the centroid to each side is rational, because for all triangles this distance is the ratio of twice the area to three times the side length.^{[15]} This can be generalized by stating that all centers associated with Heronian triangles whose barycentric coordinates are rational ratios have a rational distance to each side. These centers include the circumcenter, orthocenter, ninepoint center, symmedian point, Gergonne point and Nagel point.^{[16]}
 All Heronian triangles can be placed on a lattice with each vertex at a lattice point.^{[17]}
Exact formula for all Heronian triangles
The Indian mathematician Brahmagupta (598668 A.D.) derived the parametric solution such that every Heronian triangle has sides proportional to:^{[18]}^{[19]}
for integers m, n and k where:
 .
The proportionality factor is generally a rational ^{p}⁄_{q} where q = gcd(a, b, c) reduces the generated Heronian triangle to its primitive and p scales up this primitive to the required size. For example, taking m = 36, n = 4 and k = 3 produces a triangle with a = 5220, b = 900 and c = 5400, which is similar to the 5, 29, 30 Heronian triangle and the proportionality factor used has p = 1 and q = 180.
The obstacle for a computational use of Brahmagupta’s parametric solution is the denominator q of the proportionality factor. q can only be determined by calculating the greatest common divisor of the three sides ( gcd(a, b, c) ) and introduces an element of unpredictability into the generation process.^{[19]} The easiest way of generating lists of Heronian triangles is to generate all integer triangles up to a maximum side length and test for an integral area.
Faster algorithms have been derived by Kurz (2008).
There are infinitely many primitive and indecomposable nonPythagorean Heronian triangles with integer values for the inradius and all three of the exradii, including the ones generated by^{[20]}^{:Thm. 4}
There are infinitely many Heronian triangles that can be placed on a lattice such that not only are the vertices at lattice points, as holds for all Heronian triangles, but additionally the centers of the incircle and excircles are at lattice points.^{[20]}^{:Thm. 5}
See also formulas for Heronian triangles with one angle equal to twice another, Heronian triangles with sides in arithmetic progression, and isosceles Heronian triangles.
Examples
The list of primitive integer Heronian triangles, sorted by area and, if this is the same, by perimeter, starts as in the following table. "Primitive" means that the greatest common divisor of the three side lengths equals 1.
Area  Perimeter  side length b+d  side length e  side length c 

6  12  5  4  3 
12  16  6  5  5 
12  18  8  5  5 
24  32  15  13  4 
30  30  13  12  5 
36  36  17  10  9 
36  54  26  25  3 
42  42  20  15  7 
60  36  13  13  10 
60  40  17  15  8 
60  50  24  13  13 
60  60  29  25  6 
66  44  20  13  11 
72  64  30  29  5 
84  42  15  14  13 
84  48  21  17  10 
84  56  25  24  7 
84  72  35  29  8 
90  54  25  17  12 
90  108  53  51  4 
114  76  37  20  19 
120  50  17  17  16 
120  64  30  17  17 
120  80  39  25  16 
126  54  21  20  13 
126  84  41  28  15 
126  108  52  51  5 
132  66  30  25  11 
156  78  37  26  15 
156  104  51  40  13 
168  64  25  25  14 
168  84  39  35  10 
168  98  48  25  25 
180  80  37  30  13 
180  90  41  40  9 
198  132  65  55  12 
204  68  26  25  17 
210  70  29  21  20 
210  70  28  25  17 
210  84  39  28  17 
210  84  37  35  12 
210  140  68  65  7 
210  300  149  148  3 
216  162  80  73  9 
234  108  52  41  15 
240  90  40  37  13 
252  84  35  34  15 
252  98  45  40  13 
252  144  70  65  9 
264  96  44  37  15 
264  132  65  34  33 
270  108  52  29  27 
288  162  80  65  17 
300  150  74  51  25 
300  250  123  122  5 
306  108  51  37  20 
330  100  44  39  17 
330  110  52  33  25 
330  132  61  60  11 
330  220  109  100  11 
336  98  41  40  17 
336  112  53  35  24 
336  128  61  52  15 
336  392  195  193  4 
360  90  36  29  25 
360  100  41  41  18 
360  162  80  41  41 
390  156  75  68  13 
396  176  87  55  34 
396  198  97  90  11 
396  242  120  109  13 
Lists of primitive Heronian triangles whose sides do not exceed 6,000,000 can be found at "Lists of primitive Heronian triangles". Sascha Kurz, University of Bayreuth, Germany. Retrieved 29 March 2016.
Equable triangles
A shape is called equable if its area equals its perimeter. There are exactly five equable Heronian triangles: the ones with side lengths (5,12,13), (6,8,10), (6,25,29), (7,15,20), and (9,10,17).^{[21]}^{[22]}
Almostequilateral Heronian triangles
Since the area of an equilateral triangle with rational sides is an irrational number, no equilateral triangle is Heronian. However, there is a unique sequence of Heronian triangles that are "almost equilateral" because the three sides are of the form n − 1, n, n + 1. A method for generating all solutions to this problem based on continued fractions was described in 1864 by Edward Sang,^{[23]} and in 1880 Reinhold Hoppe gave a closedform expression for the solutions.^{[24]} The first few examples of these almostequilateral triangles are listed in the following table (sequence A003500 in the OEIS):
Side length  Area  Inradius  

n − 1  n  n + 1  
3  4  5  6  1 
13  14  15  84  4 
51  52  53  1170  15 
193  194  195  16296  56 
723  724  725  226974  209 
2701  2702  2703  3161340  780 
10083  10084  10085  44031786  2911 
37633  37634  37635  613283664  10864 
Subsequent values of n can be found by multiplying the previous value by 4, then subtracting the value prior to that one (52 = 4 × 14 − 4, 194 = 4 × 52 − 14, etc.), thus:
where t denotes any row in the table. This is a Lucas sequence. Alternatively, the formula generates all n. Equivalently, let A = area and y = inradius, then,
where {n, y} are solutions to n^{2} − 12y^{2} = 4. A small transformation n = 2x yields a conventional Pell equation x^{2} − 3y^{2} = 1, the solutions of which can then be derived from the regular continued fraction expansion for √3.^{[25]}
The variable n is of the form , where k is 7, 97, 1351, 18817, …. The numbers in this sequence have the property that k consecutive integers have integral standard deviation.^{[26]}
See also
References
 ^ Carlson, John R. (1970), "Determination of Heronian Triangles" (PDF), Fibonacci Quarterly, 8: 499–506
 ^ Beauregard, Raymond A.; Suryanarayan, E. R. (January 1998), "The Brahmagupta Triangles" (PDF), College Mathematics Journal, 29 (1): 13–17, doi:10.2307/2687630, JSTOR 2687630
 ^ Weisstein, Eric W. "Heronian Triangle". MathWorld.
 ^ ^{a} ^{b} ^{c} Yiu, Paul (2008), Heron triangles which cannot be decomposed into two integer right triangles (PDF), 41st Meeting of Florida Section of Mathematical Association of America
 ^ Sierpiński, Wacław (2003) [1962], Pythagorean Triangles, Dover Publications, Inc., ISBN 9780486432786

^ ^{a} ^{b} Friche, Jan (2 January 2002). "On Heron Simplices and Integer Embedding". ErnstMoritzArndt Universät Greiswald Publication. arXiv:math/0112239. Cite journal requires
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^ Buchholz, R. H.; MacDougall, J. A. (2001). "Cyclic Polygons with Rational Sides and Area". CiteSeerX Penn State University: 3. CiteSeerX 10.1.1.169.6336. Cite journal requires
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(help)  ^ ^{a} ^{b} Somos, M. (December 2014). "Rational triangles". Retrieved 20181104.
 ^ Mitchell, Douglas W. (2013), "Perpendicular Bisectors of Triangle Sides", Forum Geometricorum 13, 53−59: Theorem 2.

^ ^{a} ^{b} Carlson, John R. (1970). "Determination of Heronian triangles" (PDF). San Diego State College. Cite journal requires
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(help)  ^ Buchholz, R. H.; MacDougall, J. A. (1999). "Heron Quadrilaterals with sides in Arithmetic or Geometric progression". Bulletin of the Australian Mathematical Society. 59: 263–269. doi:10.1017/s0004972700032883.
 ^ Blichfeldt, H. F. (1896–1897). "On Triangles with Rational Sides and Having Rational Areas". Annals of Mathematics. 11 (1/6): 57–60. doi:10.2307/1967214. JSTOR 1967214.
 ^ Zelator, K., "Triangle Angles and Sides in Progression and the diophantine equation x2+3y2=z2", Cornell Univ. archive, 2008
 ^ Bailey, Herbert, and DeTemple, Duane, "Squares inscribed in angles and triangles", Mathematics Magazine 71(4), 1998, 278–284.
 ^ Clark Kimberling, "Trilinear distance inequalities for the symmedian point, the centroid, and other triangle centers", Forum Geometricorum, 10 (2010), 135−139. http://forumgeom.fau.edu/FG2010volume10/FG201015index.html
 ^ Clark Kimberling's Encyclopedia of Triangle Centers "Encyclopedia of Triangle Centers". Archived from the original on 20120419. Retrieved 20120617.
 ^ Yiu, P., "Heronian triangles are lattice triangles", American Mathematical Monthly 108 (2001), 261–263.
 ^ Carmichael, R. D., 1914, "Diophantine Analysis", pp.1113; in R. D. Carmichael, 1959, The Theory of Numbers and Diophantine Analysis, Dover Publications, Inc.
 ^ ^{a} ^{b} Kurz, Sascha (2008). "On the generation of Heronian triangles". Serdica Journal of Computing. 2 (2): 181–196. arXiv:1401.6150. Bibcode:2014arXiv1401.6150K. MR 2473583..
 ^ ^{a} ^{b} Zhou, Li, "Primitive Heronian Triangles With Integer Inradius and Exradii", Forum Geometricorum 18, 2018, 7177. http://forumgeom.fau.edu/FG2018volume18/FG201811.pdf
 ^ Dickson, Leonard Eugene (2005), History of the Theory of Numbers, Volume Il: Diophantine Analysis, Dover Publications, p. 199, ISBN 9780486442334
 ^ Markowitz, L. (1981), "Area = Perimeter", The Mathematics Teacher, 74 (3): 222–3
 ^ Sang, Edward, "On the theory of commensurables", Transactions of the Royal Society of Edinburgh, 23: 721–760, doi:10.1017/s0080456800020019. See in particular p. 734.
 ^ Gould, H. W. (February 1973), "A triangle with integral sides and area" (PDF), Fibonacci Quarterly, 11 (1): 27–39.
 ^ Richardson, William H. (2007), SuperHeronian Triangles
 ^ Online Encyclopedia of Integer Sequences, OEIS: A011943.
External links
 Weisstein, Eric W. "Heronian triangle". MathWorld.
 Online Encyclopedia of Integer Sequences Heronian
 Wm. Fitch Cheney, Jr. (January 1929), "Heronian Triangles", Amer. Math. Monthly, 36 (1): 22–28, JSTOR 2300173
 S. sh. Kozhegel'dinov (1994), "On fundamental Heronian triangles", Math. Notes, 55 (2): 151–6, doi:10.1007/BF02113294