Papers by P. Giannopoulos
Canadian Journal of Gastroenterology, 2007
Hydatid disease, although endemic mostly in sheep-farming countries, remains a public health issu... more Hydatid disease, although endemic mostly in sheep-farming countries, remains a public health issue worldwide, involving mainly the liver. Intrabiliary rupture is the most frequent complication of the hepatic hydatid cyst. Endoscopy is advocated, preoperatively, to alleviate obstructive jaundice caused by intracystic materials after a frank rupture and is also a useful and well-established adjunct in locating postoperative biliary fistulas.Endoscopic retrograde cholangiography with sphincterotomy has been successful as the sole and definitive means of treatment of intra-biliary ruptured hydatid cysts. A case of an elderly woman with frank rupture is presented, where the rupture was definitively managed endoscopically in conjunction with sphincterotomy to remove the intrabiliary obstructive daughter cysts and to achieve decontamination of the biliary tree.Endoscopic retrograde cholangiography provided an excellent diagnostic and therapeutic modality in the present case and, thus, it s...
World Journal of Gastroenterology
Mechanical behavior of colonic anastomosis in experimental settings as a measure of wound repair ... more Mechanical behavior of colonic anastomosis in experimental settings as a measure of wound repair and tissue integrity.
16th International Congress of the European Association for Endoscopic Surgery (EAES) Stockholm, Sweden, 11–14 June 2008
Surgical Endoscopy, 2009
Background: The published recurrence rate after laparoscopic ventral hernia repair is much less t... more Background: The published recurrence rate after laparoscopic ventral hernia repair is much less than the rate of recurrence via the open approach. Studies have demonstrated the safety and efficacy of this procedure but have had relatively young patient populations. We present our experience in a significantly older population. Methods: A retrospective chart review of all patients aged 80–89 years undergoing a laparoscopic ventral hernia repair at our institution from May, 2000 to June, 2007 was performed. Data collected included demographics, number and type of previous abdominal operations, number of previous hernia repairs, defect and mesh size, postoperative complications, and follow-up. Results: Twenty octagenerian patients underwent laparoscopic ventral hernia repair. There were 9 men and 11 women. The mean age was 82 years (80–89 yrs). Mean BMI was 29.35. Thirteen patients (65%) had one or more associated co-morbidities at the time of surgery. Eighteen patients (90%) had undergone a mean of 1.7 prior abdominal operations. Six (30 %) patients had undergone a mean of 1.1 previous open hernia repairs; 83% with mesh. Eight patients (40%) had an additional operative procedure at the time of laparoscopic hernia repair. The mean length of stay was 4.8 days (0–9 days). Mean follow-up was 106 days. Ten minor complications occurred in 10 patients. Five major complications occurred in 5 patients. One patient required reoperation for evacuation of hematoma. No patients complained of pain at a transabdominal suture site or persistent seromas by 6 weeks of follow-up. No recurrences occurred and no patients required mesh removal in this series. There were no deaths. Conclusions: The significantly elderly benefit from laparoscopic ventral hernia without any increased complications or mortality due to their advanced age or inherently increased co-morbidities. P003 Abdominal Cavity and Abdominal Wall
Pneumothorax complicating endoscopic sphincterotomy successfully treated conservatively
Acta gastro-enterologica Belgica
Milling a Graph with Turn Costs: A Parameterized Complexity Perspective
Lecture Notes in Computer Science, 2010
The Discrete Milling problem is a natural and quite general graph-theoretic model for geometric m... more The Discrete Milling problem is a natural and quite general graph-theoretic model for geometric milling problems: Given a graph, one asks for a walk that covers all its vertices with a minimum number of turns, as specified in the graph model by a 0/1 turncost function ...
On the Computational Complexity of Erdős-Szekeres and Related Problems in ℝ3
Lecture Notes in Computer Science, 2013
ABSTRACT The Erdős-Szekeres theorem states that, for every k, there is a number n k such that eve... more ABSTRACT The Erdős-Szekeres theorem states that, for every k, there is a number n k such that every set of n k points in general position in the plane contains a subset of k points in convex position. If we ask the same question for subsets whose convex hull does not contain any other point from the set, this is not true: as shown by Horton, there are sets of arbitrary size that do not contain an empty convex 7-gon. These problems have been studied also from a computational point of view, and, while several polynomial-time algorithms are known for finding a largest (empty) convex subset in the planar case, the complexity of the problems in higher dimensions has been, so far, open. In this paper, we give the first non-trivial results in this direction. First, we show that already in dimension 3 (the decision versions of) both problems are NP-hard. Then, we show that when an empty convex subset is sought, the problem is even W[1]-hard with respect to the size of the solution subset.
A Pseudo-Metric for Weighted Point Sets
Lecture Notes in Computer Science, 2002
A Pseudo-Met rie for Weighted Point Sets Panos Giannopoulos and Bemco C. Veltkamp Department of C... more A Pseudo-Met rie for Weighted Point Sets Panos Giannopoulos and Bemco C. Veltkamp Department of Computer Science, Utrecht University Padualaan 14, 3584 CH Utrecht, The Netherlands {panos,Reraco.Veltkamp}@cs.uu.nl Abstract. We derive a pseudo-metric for weighted ...
Finding a largest empty convex subset in space is W [1]-hard
ABSTRACT We consider the following problem: Given a point set in space find a largest subset that... more ABSTRACT We consider the following problem: Given a point set in space find a largest subset that is in convex position and whose convex hull is empty. We show that the (decision version of the) problem is W[1]-hard.
The Complexity of Separating Points in the Plane
Algorithmica, 2014
ABSTRACT We study the following separation problem: Given n connected curves and two points s and... more ABSTRACT We study the following separation problem: Given n connected curves and two points s and t in the plane, compute the minimum number of curves one needs to retain so that any path connecting s to t intersects some of the retained curves. We give the first polynomial (O(n3)) time algorithm for the problem, assuming that the curves have reasonable computational properties. The algorithm is based on considering the intersection graph of the curves, defining, in this graph, an appropriate family of closed walks that satisfies the 3-path-condition, and arguing that a shortest cycle in the family gives an optimal solution. The 3-path-condition has been used mainly in topological graph theory, and thus its use here reveals the connection to topology. We also show that the generalized version, where several input points are to be separated, is NP-hard for natural families of curves, like segments in two directions or unit circles.
Parameterized Complexity of Geometric Problems
The Computer Journal, 2007
Hardness of discrepancy computation and -net verification in high dimension
Journal of Complexity, 2012
ABSTRACT Discrepancy measures how uniformly distributed a point set is with respect to a given se... more ABSTRACT Discrepancy measures how uniformly distributed a point set is with respect to a given set of ranges. Depending on the ranges, several variants arise, including star discrepancy, box discrepancy, and discrepancy of halfspaces. These problems are solvable in time nO(d), where d is the dimension of the underlying space. As such a dependency on d becomes intractable for high-dimensional data, we ask whether it can be moderated. We answer this question negatively by proving that the canonical decision problems are W[1]-hard with respect to the dimension, implying that no f(d)⋅nO(1)-time algorithm is possible for any function f(d) unless FPT=W[1]. We also discover the W[1]-hardness of other well known problems, such as determining the largest empty box that contains the origin and is inside the unit cube. This is shown to be hard even to approximate within a factor of 2n.
Information Processing Letters, 2008
Deciding whether two n-point sets A, B ∈ R d are congruent is a fundamental problem in geometric ... more Deciding whether two n-point sets A, B ∈ R d are congruent is a fundamental problem in geometric pattern matching. When the dimension d is unbounded, the problem is equivalent to graph isomorphism and is conjectured to be in FPT.
Computational Geometry, 2013
We study the following general stabbing problem from a parameterized complexity point of view: Gi... more We study the following general stabbing problem from a parameterized complexity point of view: Given a set S of n translates of an object in R d , find a set of k lines with the property that every object in S is "stabbed" (intersected) by at least one line.
Computational Geometry, 2008
The Earth Mover's Distance (EMD) between two weighted point sets (point distributions) is a dista... more The Earth Mover's Distance (EMD) between two weighted point sets (point distributions) is a distance measure commonly used in computer vision for color-based image retrieval and shape matching. It measures the minimum amount of work needed to transform one set into the other one by weight transportation.
gudmundsson.biz
Abstract. Given a Euclidean graph G in Rd with n vertices and m edges, we consider the problem of... more Abstract. Given a Euclidean graph G in Rd with n vertices and m edges, we consider the problem of adding an edge to G such that the stretch factor of the resulting graph is minimized. Currently, the fastest algorithm for computing the stretch factor of a graph with positive ...
Abstract. The Abstract Milling problem is a natural and quite general graph-theoretic model for g... more Abstract. The Abstract Milling problem is a natural and quite general graph-theoretic model for geometric milling problems. Given a graph, one asks for a walk that covers all its vertices with a minimum number of turns, as specified in the graph model by a 0/1 turncost function fx at each vertex x giving, for each ordered pair of edges (e, f ) incident at x, the turn cost at x of a walk that enters the vertex on edge e and departs on edge f . We describe an initial study of the parameterized complexity of the problem. Our main positive result shows that Abstract Milling, parameterized by: number of turns, treewidth and maximum degree, is fixed-parameter tractable, We also show that Abstract Milling parameterized by (only) the number of turns and the pathwidth, is hard for W [1] -one of the few parameterized intractability results for bounded pathwidth.
We study the complexity of the following cell connection and separation problems in segment arran... more We study the complexity of the following cell connection and separation problems in segment arrangements. Given a set of straight-line segments in the plane and two points a and b:
ACM Transactions on Algorithms, 2011
We present an algorithm for the 3-center problem in (R d , L∞), i. e., for finding the smallest s... more We present an algorithm for the 3-center problem in (R d , L∞), i. e., for finding the smallest side length for 3 cubes that cover a given n-point set in R d , that runs in O(n log n) time for any fixed dimension d. This shows that the problem is fixed-parameter tractable when parameterized with d. On the other hand, using tools from parameterized complexity theory, we show that this is unlikely to be the case with the k-center problem in (R d , L2), for any k ≥ 2. In particular, we prove that deciding whether a given n-point set in R d can be covered by the union of 2 balls of given radius is W[1]-hard with respect to d, and thus not fixed-parameter tractable unless FPT=W[1]. Our reduction also shows that even an O(n o(d) )-time algorithm for the latter does not exist, unless SNP ⊂ DTIME(2 o(n) ).
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Papers by P. Giannopoulos