1. Piaget J., The Child's Conception of Number (Routledge & Kegan Paul, London, 1952).
2. Perception of Numbers by Human Infants
3. Addition and subtraction by human infants
4. Wynn K., Math. Cognit.1, 35 (1995).
5. Butterworth B., The Mathematical Brain (Macmillan, London, 1999).