Density of Binary Disc Packings: Lower and Upper Bounds

We provide, for any r∈(0,1), lower and upper bounds on the maximal density of a packing in the Euclidean plane of discs of radius 1 and <i>r</i>. The lower bounds are mostly folk, but the upper bounds improve the best previously known ones for any r∈[0.11,0.74]. For many values of <i>r</i>, this gives a fairly good idea of the exact maximum density. In particular, we get new intervals for <i>r</i> which does not allow any packing more dense that the hexagonal packing of equal discs.