In (15.6.2) the point lies outside the integration
contour, and assume their principal values where the
contour cuts the interval , and at .
In (15.6.3) the point lies outside the integration
contour, the contour cuts the real axis between and , at which
point and .
In (15.6.5) the integration contour starts and terminates at a point
on the real axis between and . It encircles and once
in the positive direction, and then once in the negative direction. See Figure
15.6.1. At the starting point and are zero.
If desired, and as in Figure 5.12.3, the upper integration limit in
(15.6.5) can be replaced by . However, this reverses the
direction of the integration contour, and in consequence (15.6.5) would
need to be multiplied by .