Roulette (curve) in this case, the point lies on the circle then the roulette is a cycloid. A related concept is a glissette, the curve described by a point attached to a given curve as it slides along two (or more) given curves. Formally speaking, the curves must be differentiable curves in the Euclidean plane. The "fixed curve" is kept invariant; the "rolling curve" is subjected to a continuous congruence transformation such that at all times the curves are tangent at a point of contact that moves with the same speed when taken along either curve (another way to express this constraint
Elliptic curve point multiplication and "Q", addition is defined as the negation of the point resulting from the intersection of the curve, "E", and the straight line defined by the points "P" and "Q", giving the point, "R". Assuming the elliptic curve, "E", is given by , this can be calculated as: Where the points "P" and "Q", are coincident (at the same coordinates), addition is similar, except that there is no well-defined straight line through "P" and "Q", so the operation is closed using limiting case, the tangent to the curve, "E", at "P" and "Q". This is calculated as above, except with: