Real Geometry and Connectedness via Triangular Description

Project: Research council


Connectedness, as in "can we get there from here", is a fundamental concept, both in actual space and in various abstract spaces. Consider a long ladder in a right-angled corridor: can it get round the corner? Calling it a corridor implies that it is connected in actual three-dimensional space. But if we consider the space of configurations of the ladder, this is determined by the position and orientation of the ladder, and the `corridor' is now the requirement that no part of the ladder run into the walls - it is not sufficient that the ends of the ladder be clear of the walls. If the ladder is too long, it may have two feasible positions, one in each arm of the corridor, but there may be no possible way to get from one to the other. In this case we say that the configuration space of the ladder is not connected: we can't get the ladder there from here, even though we can get each end (taken separately, which is physically impossible) from here to there. Connectedness in configuration space is therefore the key to motion planning. These are problems human beings (especially furniture movers, or people trying to park cars in confined spaces) solve intuitively, but find very hard to explain. Note that the ladder is rigid and three-dimensional, hence its position is determined by the coordinates of three points on it, so configuration space is nine-dimensional. Connectedness in mathematical spaces is also important. The square root of 4 can be either 2 or -2: we have to decide which. Similarly, the square root of 9 can be 3 or -3. But, if 4 is connected to 9 in our problem space (whatever that is), we can't make these choices independently: our choice has to be consistent along the path from 4 to 9. When it is impossible to make such decisions totally consistently, we have what mathematicians call a `branch cut' - the classic example being the International Date Line, because it is impossible to assign `day' consistently round a globe. In previous work, we have shown that several mathematical paradoxes reduce to connectedness questions in an appropriate space divided by the relevant branch cuts. This is an area of mathematics which is notoriously difficult to get right by hand, and mathematicians, and software packages, often have internal inconsistencies when it comes to branch cuts. The standard computational approach to connectedness, which has been suggested in motion planning since the early 1980s, is via a technique called cylindrical algebraic decomposition. This has historically been computed via a "bottom-up" approach: we first analyse one direction, say the x-axis, decomposing it into all the critical points and intermediate regions necessary, then we take each (x,y)-cylinder above each critical point or region, and decompose it, then each (x,y,z) above each of these regions, and so on. Not only does this sound tedious, but it is inevitably tedious - the investigators and others have shown that the problem is extremely difficult (doubly exponential in the number of dimensions). Much of the time, notably in motion planning, we are not actually interested in the lower-dimensional components, since they would correspond to a motion with no degrees of freedom, rather like tightrope-walking. Recent Canadian developments have shown an alternative way of computing such decompositions via so-called triangular decompositions, and a 2010 paper (Moreno Maza in Canada + Davenport) has shown that the highest-dimensional components of a triangular decomposition can be computed in singly-exponential time. This therefore opens up the prospect, which we propose to investigate, of computing the highest-dimensional components of a cylindrical decomposition in singly-exponential time, which would be a major breakthrough in computational geometry.
Effective start/end date1/10/1131/12/15


critical point
computational geometry
degrees of freedom
computer programs