is physically correlated, ranging from $\sim200$ to $\sim1000$ pc in different galaxies. Numerical simulations investigating the propagation of star formation in a turbulent medium derive an age-separation relation to the 0.5 power for star groups with separations $>50$ pc [@nomura2001], which is in agreement with the observational data.

The relation between age difference and separation for star clusters is also similar to the correlation between size and line width of giant molecular clouds (GMC) suggesting that the crossing time in a GMC increases as the square root of the size of the star formation region [@larson1981; @elmegreen1996].

In this article, we develop an alternative method for studying processes of triggered star formation. In @gusev2018 we found, but only noted a dependence between separations of star clusters from the nearest H[ii]{} region and $U-B$ colour index of the star cluster for the pairs with non-deprojected angular separation from 1.5 to 7 arcsec. This study is devoted to the analysis of this dependence. As is known, $U-B$ colour index can be used as an age indicator for young stellar systems [@whitmore2010].

The youngest star formation regions with ages less than 10 Myr are usually associated with H[ii]{} regions. They evolves during the 