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The set S = {42} has 42 as both an upper bound and a lower bound; all other numbers are either an upper bound or a lower bound for that S. Every subset of the natural numbers has a lower bound since the natural numbers have a least element (0 or 1, depending on convention). An infinite subset of the natural numbers cannot be bounded from above.
Formulae [9] and fast algorithms [10] are known for three numbers though the calculations can be very tedious if done by hand. Simpler lower and upper bounds for Frobenius numbers for n = 3 have also been determined. The asymptotic lower bound due to Davison
The best known upper bound on the size of a square-difference-free set of numbers up to is only slightly sublinear, but the largest known sets of this form are significantly smaller, of size . Closing the gap between these upper and lower bounds remains an open problem.
But this is just the least element of the whole poset, if it has one, since the empty subset of a poset P is conventionally considered to be both bounded from above and from below, with every element of P being both an upper and lower bound of the empty subset. Other common names for the least element are bottom and zero (0).
The exact MILP results for 3,4,5,6,7 correspond to the lower bound. For k>7, no exact results are known, but the difference between the lower and upper bound is less than 0.3%. When the parameter is the number of subsets (m), the approximation ratio is exactly .
An upper bound for R(r, s) can be extracted from the proof of the theorem, and other arguments give lower bounds. (The first exponential lower bound was obtained by Paul Erdős using the probabilistic method.) However, there is a vast gap between the tightest lower bounds and the tightest upper bounds.
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