People competent in mathematics have been shown to have higher activation of the left angular gyrus according to fMRI. EEG recordings have shown larger activity in the posterior parietal cortex.
I think you are interested in what makes a good mathematician. A brain imaging study by Grabner et al. (2011). showed that the left anterior gyrus was more activated in individuals more competent in math. Specifically, the authors argue that the stronger left AG activity in the more competent subjects reflects a higher proficiency in processing mathematical symbols. The anterior gyrus is involved in mathematical symbol processing.
Left angular gyrus, lateral view. Source: Kenhub Neuroanatomy Atlas
ERP studies (event-related potentials deduced from the EEG) have shown higher activity in posterior parietal cortex in subjects excelling in math (Waisman et al., 2014). This region is associated with algebra and handling mathematical functions.
Posterior parietal cortex among other regions not relevant to the question answer. Source: Live Science
Grabner et al. Front Hum Neurosci 2011; 5: 130
Waisman et al., Int J Sci Math Edu 2014; 12(3): 669-96