Arnawa, I. M., sumarno, U., Kartasasmita, B. and Baskoro, E. T. (2012) “APPLYING THE APOS THEORY TO IMPROVE STUDENTS ABILITY TO PROVE IN ELEMENTARY ABSTRACT ALGEBRA”, Journal of the Indonesian Mathematical Society, 13(1), pp. 133–148. doi: 10.22342/jims.13.1.80.133-148.