Optimal Embedded and Enclosing Isosceles Triangles

Given a triangle [Formula: see text], we study the problem of determining the smallest enclosing and largest embedded isosceles triangles of [Formula: see text] with respect to area and perimeter. This problem was initially posed by Nandakumar [17, 22] and was first studied by Kiss, Pach, and Somlai [13], who showed that if [Formula: see text] is the smallest area isosceles triangle containing [Formula: see text], then [Formula: see text] and [Formula: see text] share a side and an angle. In the present paper, we prove that for any triangle [Formula: see text], every maximum area isosceles triangle embedded in [Formula: see text] and every maximum perimeter isosceles triangle embedded in [Formula: see text] shares a side and an angle with [Formula: see text]. Somewhat surprisingly, the case of minimum perimeter enclosing triangles is different: there are infinite families of triangles [Formula: see text] whose minimum perimeter isosceles containers do not share a side and an angle with [Formula: see text].