is the focal length of a convex lens negative
Focal length For a thin lens in air, the focal length is the distance from the center of the lens to the principal foci (or focal points) of the lens. For a converging lens (for example a convex lens), the focal length is positive, and is the distance at which a beam of collimated light will be focused to a single spot. For a diverging lens (for example a concave lens), the focal length is negative, and is the distance to the point from which a collimated beam appears to be diverging after passing through the lens.
Limit of a function (the Dirichlet function) has no limit at any x-coordinate.
Z-transform The Z-transform can be defined as either a one-sided or two-sided transform.[7]
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