Geometric aspects of effectively hyperbolic critical points on time $t=0$ are discussed assuming that the characteristic roots are real on one side of time $t$, namely time is positive. In particular, we aim to elucidate the differences in the geometric aspects of effectively hyperbolic critical points on time $t=0$ when the characteristic roots are real on both the positive and negative sides of time.
References
V. Ivrii and V. M. Petkov, Necessary conditions for the correctness of the Cauchy problem for non-strictly hyperbolic equations, Uspehi Mat. Nauk, 29 (1974), no. 5, 3-70.V. Ivrii and V. M. Petkov, Necessary conditions for the correctness of the Cauchy problem for non-strictly hyperbolic equations, Uspehi Mat. Nauk, 29 (1974), no. 5, 3-70.
T. Nishitani, The Cauchy problem for operators with triple effectively hyperbolic characteristics: Ivrii's conjecture, J. Anal. Math., 149 (2023), no. 1, 167-237.T. Nishitani, The Cauchy problem for operators with triple effectively hyperbolic characteristics: Ivrii's conjecture, J. Anal. Math., 149 (2023), no. 1, 167-237.