who can offer me a site about nodes?

Answer 1

You need to be a little more specific.
Nodes can occur in graph theory, information theory, wave theory, and several other areas. Also, these areas are commonly taught at different levels. When I search on nodes math here are a few of the sites that turn up:
http://www.math.tu-berlin.de/geometrie/f5/wiki/index.php/Nodes
http://mathworld.wolfram.com/Graph.html
http://www-math.mit.edu/18085
http://groups.google.com/group/sci.math/msg/8585e5ec075de382
http://mathworld.wolfram.com/CatalanNumber.html
If I search only on nodes without the additional keyword math, I also turn up sites related to transit, botany, lymph nodes, lunar nodes, nodes in standing waves (physics) etc. Here's an example --
http://en.wikipedia.org/wiki/Node_(physics)