a convective core ($N^2 < 0$) in the model on the blue loop, whereas the model that undergoes shell hydrogen burning has a radiative core. Beyond the core, the propagation diagrams of these two models are qualitatively similar; in particular, both have a fully developed intermediate convective zone. The two spikes in the run of Brunt-Väisälä frequency that occur in the vicinity of convective zones in Model2 have different origin. The one at the border of the convective core is both a consequence of the very steep $\mu$-gradient in this region of star (the main reason) and also a numerical effect related to the extreme difficulties with precise determination of the border of helium core by evolution codes. Due to a very high sensitivity of triple-alpha process to a change of temperature, the location of border changes slightly with every step of evolution. That results in an artificial increase of the $\mu$-gradient, which is clearly visible in behaviour of $N^2$. The second spike, at the innermost border of ICZ is related to the presence of inward overshooting from this convective zone. That leads to a steep change of $\mu$ and a peak in the propagation diagram.

In Fig.\[fig4\], we present 