Let u' = Au be the linear approximation of the following system of non linear differential equation
x'=F(x,y); y'=G(x,y)
Let X1 X2 be two eigenvalues of the 2 x 2 matrix A at the critical point (xo,yo). Then
A. (xo, Yo) is an improper node if X1,X2 are real and have same sign B. (xo, Yo) is a spiral point if X1, X2 are complex numbers. C. (o,Yo) is an proper/improper node if X1,X2 are real and equal D. (xo, Yo) is a saddle point if X1, X2 are real and unequal. E. (o,Yo) is unstable if X1,X2 are purely imaginary