Estimating the Fundamental Limits is Easier than Achieving the Fundamental Limits

We show through case studies that it is easier to estimate the fundamental limits of data processing than to construct explicit algorithms to achieve those limits. Focusing on binary classification, data compression, and prediction under logarithmic loss, we show that in the finite space setting, when it is possible to construct an estimator of the limits with vanishing error with n samples, it may require at least nln n samples to construct an explicit algorithm to achieve the limits.