Since we know the price will increase after trading, due to slippage, and in order to make the SOR EVM-tractable, the function that calculates the spot price of a Balancer pool after a trade has been linearized. We refer to EP – the estimated price – as the spot price a pool will have after a trade, according to that linearized approximation. The figure below shows the real (non-linear) spot price after a given amount is traded versus the linearized approximation we use.