wherein x is an orthogonal coordinate along a pitch line of a rack, y is an orthogonal coordinate along a depth of the rack, m is a module of the tooth, n is 1/2 of the number of tooth difference between the rigid internal gear and the flexible externalgear, η is an angular parameter, κ is a deflection coefficient (κ = 1 represents a state of non deviation,whereas κ < 1 represents that of negative deviation).