## Proper or improper node?

Nonlinear differential equations and stability. For eigenvalues of the form r_1 = r_2 we have two cases, either r_1 > 0 or r_1 < 0, which affects the stability. However we also have that the critical point may be either a proper or improper node. How is this determined? Under what circumstances is the node proper, and under what circumstances is the node improper?