Suppose that "C" is a smooth algebraic curve of genus "g". If "g" is zero, then "C" is P1, and the canonical class is the class of −2"P", where "P" is any point of "C". This follows from the calculus formula "d"(1/"t") = −"dt"/"t"2, for example, a meromorphic differential with double pole at the point at infinity on the Riemann sphere. In particular, "K""C" and its multiples are not effective. If "g" is one, then "C" is an elliptic curve, and "K""C" is the trivial bundle. The global sections of the trivial bundle form a one-dimensional vector space, so the "n"-canonical map for any "n" is the map to a point.