Moving sofa problem

Moving sofa problem
In mathematics, the moving sofa problem or sofa problem is a two-dimensional idealisation of real-life furniture-moving problems and asks for the rigid two-dimensional shape of largest area that can be maneuvered through an L-shaped planar region with legs of unit width. The area thus obtained is referred to as the sofa constant. The exact value of the sofa constant is an open problem. The currently leading solution, by Joseph L. Gerver, has a value of approximately 2.2195 and is thought to be close to the optimal, based upon subsequent study and theoretical bounds.
History
The first formal publication was by the Austrian-Canadian mathematician Leo Moser in 1966, although there had been many informal mentions before that date.
Bounds
Work has been done on proving that the sofa constant (A) cannot be below or above certain values (lower bounds and upper bounds).
Lower
Lower bounds can be proven by finding a specific shape of high area and a path for moving it through the corner. An obvious lower bound is . This comes from a sofa that is a half-disk of unit radius, which can slide up one passage into the corner, rotate within the corner around the center of the disk, and then slide out the other passage.
In 1968, John Hammersley stated a lower bound of . This can be achieved using a shape resembling a telephone handset, consisting of two quarter-disks of radius 1 on either side of a 1 by rectangle from which a half-disk of radius has been removed.
In 1992, Joseph L. Gerver of Rutgers University described a sofa specified by 18 curve sections each taking a smooth analytic form. This further increased the lower bound for the sofa constant to approximately 2.2195 .
Upper
Hammersley stated an upper bound on the sofa constant of at most . Yoav Kallus and Dan Romik published a new upper bound in 2018, capping the sofa constant at . Their approach involves rotating the corridor (rather than the sofa) through a finite sequence of distinct angles (rather than continuously), and using a computer search to find translations for each rotated copy so that the intersection of all of the copies has a connected component with as large an area as possible. As they show, this provides a valid upper bound for the optimal sofa, which can be made more accurate by using more rotation angles. A set of five carefully-chosen rotation angles leads to the stated upper bound.
Ambidextrous sofa
A variant of the sofa problem asks the shape of largest area that can go round both left and right 90 degree corners in a corridor of unit width (where the left and right corners are spaced sufficiently far apart that one is fully negotiated before the other is encountered). A lower bound of area approximately 1.64495521 has been described by Dan Romik. His sofa is also described by 18 curve sections.
See also
Dirk Gently's Holistic Detective Agency – novel by Douglas Adams, a subplot of which revolves around such a problem.
Moser's worm problem
Square packing in a square
"The One with the Cop" - an episode of the American TV series Friends with a subplot revolving around such a problem.
References
External links
SofaBounds - Program to calculate bounds on the sofa moving problem.
A 3D model of Romik's ambidextrous sofa
Discrete geometry
Unsolved problems in geometry
Recreational mathematics
1966 introductions