A Density-Based Extension of Supply and Demand: Modeling Market Participation in 3D

This work proposes a three-dimensional extension of classical supply and demand theory by introducing market participation density as a third axis. It models demand and supply not as lines but as density surfaces over price–quantity space, enabling a richer understanding of market dynamics, especially in cases of crisis, asymmetry, or low liquidity. A novel concept called transaction density is defined as the overlap of supply and demand density functions. This leads to the definition of density surplus—a more realistic counterpart to traditional consumer and producer surplus measures. The framework is applied to the Great Depression, illustrating how demand collapse can paralyze market transactions even when supply remains active. The model bridges economic intuition with quantitative structure and provides a foundation for more robust analysis of market participation, liquidity, and breakdowns.