12 Mar
2013
12 Mar
'13
3:33 p.m.
Let d_n be the greatest density achieved by any subset of Z^2, no two points of which are closer in Euclidean distance than sqrt(n). What is known about d_n? Is it known to be always rational? I can use a compactness argument to show that the supremum density is achieved (that's why I felt free to write "the greatest density" above) but I can't prove that it's achieved by a doubly-periodic set (which would imply rational density). I get n d_n approx 2/sqrt(3) for large n using a back-of-the-envelope calculation, but I may have mis-programmed the envelope, especially since it's not an envelope but rather a bag for a pizza-slice-to-go --- a medium with no track-record as a calculation-aid. Jim Propp