Thursday, 12 July 2012

co.combinatorics - Algorithmic aspects of maximizing a convex function over a convex set

Motivation



The problem I am facing can be considered a variant of the standard set packing problem. However; instead of being given a list of sets, I am given a function nu:2Nrightarrow0,1 and want to find a partitioning P of N that maximizes g(P)=sumSinPnu(S). This can be shown to require somewhere between O(2|N|) and O(3|N|) operations.



The above problem can (almost) be reduced to the problem of finding a partition P3 of N into three sets that maximizes g(P3). However there are still O(3|N|) partitionings of N into three sets.



Lets say we construct such a 3 partition (S1,S2,S3) as follows: For each element iinN, we add i to the first set with probability xi, to the second set with probability yi and to the third with probability zi, where xi+yi+zi=1 and 0leqxi,yi,zi.



It can be shown that the expected value of such a probability distribution, x,y,z, over the 3 partitions of N is f(x,y,z)=E[g(P)]=sumcsubsetNnu(C)left[PiiinCxiPiinotinC(yi+zi)+PiiinCyiPiinotinC(xi+zi)+PiiinCziPiinotinC(xi+yi)right].



I am considering the situation in which we relax the constraint xi+yi+zi=1 to xi+yi+zileq1 and then using techniques akin to interior point methods for standard convex programming.



This relaxation clearly does not change maximum of f(x,y,z) and with it in place f(x,y,z) can be shown to be convex over our feasible set 0leqxi,yi,zi and xi+yi+zileq1 for iinN.




Given the above, my question is: Is there any general theory for maximizing convex functions over convex compact sets? (Apart from that the maximum must appear on the boundary?)



First time poster, so my apologies if I have tagged this inappropriately.
I know of much work in convex programming (minimizing convex functions over convex sets) but haven't been able to find similar work for maximization.

No comments:

Post a Comment