ABSTRACT
Let PO be a point on a curve y and U C y some neighborhood of PO such that PO decomposes U into two half-neighborhoods U l and U2 (see Figure 4.1). Take another point Q E y and consider the ray PoQ, whose origin is PO. Assume that Q , which is situated in U;, tends to PO. Then the limit position of the ray PoQ, if it exists, is called the ray tangent to y at PO with respect to O;. If there exist both rays tangent to y at PO and they form a straight line, then this line is called the straight line tangent to y at P. or the tangent to y at PO.
