Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
A ribbon with central axis curve C1 and boundary curve C2. The unit tangent to the central axis t and the unit normal vector n pointing from C1 to C2 are indicated. This particular ribbon contains a twist Tw = 1. To see this figure in color, go online.
