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Mind School·Wonder·Honor-system

The Inscribed Square Problem

Draw any closed curve. Can you always find four points on it that form a square?

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Characterization

The Inscribed Square Problem, posed by Otto Toeplitz in 1911, asks whether every simple closed curve in the plane — no matter how jagged, kinked, or wild — contains four points that form the vertices of a square. The conjecture has been proved for convex curves, for smooth curves, for piecewise-linear curves, and for curves with various regularity conditions. In 2020, Joshua Evan Greene and Andrew Lobb made a striking advance using techniques from symplectic geometry: they proved that every smooth simple closed curve inscribes a rectangle of every aspect ratio — a result stronger than the square conjecture in the smooth case. But the general continuous case — curves that may be nowhere differentiable, fractal-like, or otherwise pathological — remains open after more than a century. The problem has a deceptive visual simplicity. Draw any loop. Find a square on it. For well-behaved curves, this is easy; for arbitrary continuous curves, it is a problem that has resisted the combined tools of topology, analysis, and algebraic geometry. The Academy hosts the Inscribed Square Problem in the Mind School because it is a puzzle that begins as play — draw a curve, hunt for the square — and ends at the frontier of modern mathematics. The loop is the game board; the square is the quarry.

Lineage

Otto Toeplitz, conjecture posed in 1911; the earliest partial result by Arnold Emch (1913, 1916) for convex curves. L. G. Shnirelman proved the case for smooth enough curves (1929). Walter Stromquist, "Inscribed Squares and Square-Like Quadrilaterals in Closed Curves," Mathematika 36(2), 1989. Joshua Evan Greene and Andrew Lobb, "The Rectangular Peg Problem," Annals of Mathematics 194(2), 2021. Benjamin Matschke, "A Survey on the Square Peg Problem," Notices of the AMS 61(4), 2014, provides a comprehensive survey of partial results.

Quests

Three quests — one for each archetype. Choose the one that fits your way of taking up the discipline.

  • Draw or generate five distinct simple closed curves of varying character — convex, smooth, piecewise-linear, and at least one with an irregular or jagged boundary. For each curve, find (by measurement, construction, or computation) four points that form an inscribed square, or attempt to and document the difficulty. Record the curves, the inscribed squares found, and a reflection on which classes of curves made the search easy and which made it hard.

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  • Draw a simple closed curve on paper — not a circle or an ellipse, but something irregular. Then, using a compass and straightedge (or careful measurement), find four points on the curve that form a square. Verify the square by measuring the four side lengths and the two diagonals. Record the curve, the four points, and the measurements.

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  • Explain the inscribed square problem to a reader who knows what a closed curve is. State the conjecture; note that it has been proved for convex and smooth curves but remains open for arbitrary continuous curves. Cite Toeplitz (1911), Schnirelmann (1944), and Greene and Lobb's 2020 result on rectangles of every aspect ratio. Explain the surprise that the proof of the smooth case required symplectic geometry — a tool from mathematical physics — and what this suggests about the depth hidden in a visually simple question.

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