In November 2008, just after my son Nikolay was born, I have discovered a remarkable article by Oene Bottema that was almost forgotten. Seven years after it is available online:
http://www.sciencedirect.com/science/article/pii/S1385725856500546
A crucial role has been played by the MR0083250 (18,684g) written by D. C. Lewis Jr.:
“A short proof of the Routh-Hurwitz condition for stability in the special case of two degrees of freedom. The author considers geometrically the discontinuity arising in connection with the case of neutral stability, in which all four roots are pure imaginary.”
Dr. Oleg N. Kirillov 