Torsion in Groups of Integral Triangles

Let 0γ＜π be a fixed pythagorean angle. We study the abelian group Hr of primitive integral triangles (a,b,c) for which the angle opposite side c is γ. Addition in Hr is defined by adding the angles β opposite side b and modding out by π-γ. The only Hr for which the structure is known is Hπ/2, which is free abelian. We prove that for generalγ, Hr has an element of order two iff 2(1- cosγ) is a rational square, and it has elements of order three iff the cubic (2cosγ)x3-3x2+1=0 has a rational solution 0x1. This shows that the set of values ofγ for which Hr has two-torsion is dense in [0, π], and similarly for three-torsion. We also show that there is at most one copy of either Z2 or Z3 in Hr. Finally, we give some examples of higher order torsion elements in Hr.

Conflicts of Interest

The authors declare no conflicts of interest.

Cite this paper

W. Murray, "Torsion in Groups of Integral Triangles," Advances in Pure Mathematics, Vol. 3 No. 1, 2013, pp. 116-120. doi: 10.4236/apm.2013.31015.

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