Kurniawan, D., K. Kartono, and H. Suyitno. “The Mathematics Communication Skill Reviewed from Self-Efficacy on Group Investigation Learning With Demonstration Feedback”. Unnes Journal of Mathematics Education Research, Vol. 10, no. 2, Dec. 2021, pp. 151-6, http://journal.unnes.ac.id/sju/ujmer/article/view/47850.