The aim of the thesis is to gain knowledge concerning what pupils aged 8 and 9 need to learn to become familiar with negative numbers. The framework used in this research, variation theory, impliesthat students' problems in learning what was intended may have to do with the fact that some critical aspects of the studied object have not yet been discerned by the student. To get the pupils to understand the idea behind each critical aspect, carefully constructed examples were used. According to variation theory it is necessary to experience differences before you experience similarities. To answer the research question data was collected by using the learning study model. It is characterized by an iterative design where I as a researcher collaborate with teachers to try to find and orchestrate the critical aspects. The method is interventionist, which means that interventions are done in teaching. In the learning study I have cooperated with two primary school teachers and 64 pupils in four different classes. The data consists of video-recordings of lessons, pre- and posttests, interviews with pupils and notes from the meetings of the learning study group. When planning lessons as well as analyzing data, concepts relating to the theory of variation have been used as analytical tools.

This thesis contributes to research by investigating in detail what aspects students need to differentiate in order to become familiar with negative numbers. The results show that the pupils needed not only to discern, but also to differentiate three different critical aspects:

To differentiate the values of two negative numbers.

To differentiate the function of the minuend versus the function of the subtrahend in a subtraction.

To differentiate the minus sign for negative numbers versus the minus sign for subtraction.