The idea of storing some estimates separately and then combini n g them with samples is a good one and is also used in Gradient-TD methods. Gradient-TD methods estimate and store the product of the second two factors in (11.27) . These factors are a d ⇥ d matrix and a d -vector , so their pro du ct is j us t a d -vector , l ike w itself. We denote t h is second learned vector as v: v ⇡ E ⇥ x t x > t ⇤ 1 E[⇢ t t x t ]