Wojdat, M., R. Kanupriya, P. Janowska, B. Wolska, R. Stępniak, and M. Adam. “Comparative Analysis of Optimism and Life Satisfaction in Brazilian Male and Female Ju-Jitsu Society”. Journal of Education, Health and Sport, vol. 7, no. 12, Dec. 2017, pp. 139-54, https://apcz.umk.pl/JEHS/article/view/5073.