 Therefore, the number of regular numbers that are at most "N" can be estimated as the volume of this tetrahedron, which is
Even more precisely, using big O notation, the number of regular numbers up to "N" is
and it has been conjectured that the error term of this approximation is actually formula_5.A similar formula for the number of 3-smooth numbers up to "N" is given by Srinivasa Ramanujan in his first letter to G. H. Hardy.In the Babylonian sexagesimal notation, the reciprocal of a regular number has a finite representation, thus being easy to divide by.