is the pressure scale height at that location, $z$ is the distance in the radiative layer away from that location, and $f$ is an adjustable parameter, which we set to $0.01$ for most of calculated models. This value of $f$ mimics the behaviour of step overshooting with $\alpha_\mathrm{ov}\approx 0.1H_P$.

In our models the core helium ignition commences on the way towards the RGB but at this stage the energy produced from the helium burning is much lower than the energy produced from the hydrogen shell burning. The shell dominates the evolution up to the tip of RGB. At this point the energy produced in a helium core becomes comparable to the energy from the shell and the luminosity of a star drops. A star might eventually move towards higher effective temperatures and form a blue loop. The behaviour of the blue loops is still very poorly known and it is difficult to predict the effect of different parameters on their properties. We found that the convective overshooting has the most visible effect on their emergence, especially the inward overshooting from the non-burning zone. Without that overshooting it is impossible to obtain blue loops in our models independently of metallicity, opacity, 