KarlovaO. O., and MykhailyukV. V. “Functions of the First Baire Class With Values in Metrizable Spaces”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 58, no. 4, Apr. 2006, pp. 568–572, https://umj.imath.kiev.ua/index.php/umj/article/view/3475.