the typical age of star clusters in the range, that the power law is calculated over, 40-500 pc, is from 10 to 100-300 Myr. This result is in good agreement with previous studies of dependence between age difference and separation of cluster pairs, which gave correlations for star clusters with ages up to $\sim100$ Myr [@efremov1998; @grasha2017].

Velocity of propagation of star formation
-----------------------------------------

Data obtained in Section \[sect:age\_sep\] enable us to directly determine variations of star formation propagation velocity: $V = dS/dt$. The velocity is $$V{\rm [km\,s^{-1}]} = 
\gamma^{-1/\alpha}\alpha^{-1}t{\rm [Myr]}^{1/\alpha-1}
\label{equation:v1t}$$ or $$V{\rm [km\,s^{-1}]} = (\alpha\gamma)^{-1}S{\rm [pc]}^{1-\alpha},
\label{equation:v2s}$$ where $\gamma=10^{\beta-6}$, and $\alpha$, $\beta$ are coefficients from equation (\[equation:ts\_gen\]).

We calculated median velocities, $V_{\rm med}$, for SC-H[ii]{}R pairs using equations (\[equation:v1t\]), (\[equation:v2s\]). These values are presented in Table \[table:coeff\]. We also estimated velocity errors, $\Delta V$, based on the coefficients $\alpha$, $\beta$, and their uncertainties from Table \[table:coeff\]. Significant part of SC-H[ii]{}R pair samples has extremely large velocity estimation errors, $\Delta V > V_{\rm med}$. These estimates are not shown in the table.

The data show the approximately constant or slightly falling velocity of star formation propagation in three regular structure galaxies (NGC 628, NGC 3184, and NGC 6946) with 