The Loeschian numbers (OEIS A003136) are numbers of the form , where and are integers. For example, , with (28,2,4) as the related Loeschian triplet.

But first, something else. A different sequence is the Farey sequence, which at order 5 is

.

In two dimensions, the Farey sequence can make Ford circles, each with the number as a radius, centered above their number, and tangent to the zero line and each other.

Loeschian spheres can be made, each with the number as a curvature (the reciprocal of the radius). With as a Loeschian triplet satisfying , the sphere centers are at one of six locations.