The main focus of this paper is a pair of new approximation algorithms for
certain integer programs. First, for covering integer programs {min cx: Ax >=
b, 0 <= x <= d} where A has at most k nonzeroes per row, we give a
k-approximation algorithm. (We assume A, b, c, d are nonnegative.) For any k >=
2 and eps>0, if P != NP this ratio cannot be improved to k-1-eps, and under the
unique games conjecture this ratio cannot be improved to k-eps. One key idea is
to replace individual constraints by others that have better rounding
properties but the same nonnegative integral solutions; another critical
ingredient is knapsack-cover inequalities. Second, for packing integer programs
{max cx: Ax <= b, 0 <= x <= d} where A has at most k nonzeroes per column, we
give a (2k^2+2)-approximation algorithm. Our approach builds on the iterated LP
relaxation framework. In addition, we obtain improved approximations for the
second problem when k=2, and for both problems when every A_{ij} is small
compared to b_i. Finally, we demonstrate a 17/16-inapproximability for covering
integer programs with at most two nonzeroes per column.Comment: Version submitted to Algorithmica special issue on ESA 2009. Previous
conference version: http://dx.doi.org/10.1007/978-3-642-04128-0_