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) Conservative approximation of a ball whose center can lie within a certain neighborhood as a ball with a reduced radius. (b) Illustration of the ball “reduction” on a water molecule model represented as three balls (red, oxygen; gray, hydrogen).
