What this block carriesFour theorems on trajectories, written out: the recursion, the odometer, the kinetic identity, the driven kernel.
The normalised Feynman–Kac recursion
η0 = μ0, ηt+1 = Tft(ηtMt)
Transition through the kernel, then select through the drive, then normalise. Repeat. The terminal ηT is the terminal marginal of the path law TFQ. The state-space recursion that filtering already uses is the marginal shadow of one path-space change of measure. The kernel may keep the represented support or carry mass into states the previous marginal did not reach. The second case is generation.
§33↗ · Thm 33.1↗
The path odometer, and its endpoint split
DKL(P‖Q) = DKL(P0‖Q0) + Σt EP[ DKL(P(dxt+1|X0:t) ‖ Q(dxt+1|X0:t)) ]
DKL(P‖Q) = DKL(ρT‖μT) + Ex∼ρT[ DKL(P(dω|XT=x) ‖ Q(dω|XT=x)) ]
−EP[V(XT)] + τ DKL(P‖Q) = H(ρT) + C(ρT‖μT) + K(P‖Q; XT)
The first line is the odometer: the initial departure plus every conditional departure paid along the route. The second reads the same total from the endpoint instead. Substitute one into the objective and three terms fall out. H prices the endpoint, C its departure from the reference terminal law, K the route information the endpoint does not fix. The figure above draws two path laws that agree on H and C and differ only in K.
§34↗ · Thm 34.1↗ · Thm 34.2↗ · Cor. 34.3↗
Girsanov, and the quarter in front of it
dXt = bt(Xt) dt + √2 dWt versus dXt = (bt(Xt) + ut) dt + √2 dWt
DKL(Pu‖Q) = ¼ EPu ∫0T ‖ut‖² dt
Standard existence and Novikov conditions apply. The quarter is not a universal constant. It is 1/2σ² at the paper’s σ² = 2 convention. It moves with the diffusion coefficient. What the identity fixes is the shape: path divergence is quadratic in the control, which is why the kinetic cost inherited from the economics paper and the information price here are one quantity in two coats.
§35↗ · Thm 35.1↗
The Doob kernel
hT(x) = eg(x), ht(x) = ∫ Mt(dy | x) ht+1(y)
Mht(dy | x) = Mt(dy | x) · ht+1(y) / ht(x)
μh0(dx) = h0(x) μ0(dx) / Eμ0[h0]
Where ht is positive and finite, the path tilt by a terminal potential is itself Markov, with that initial law and that kernel. Multiply the reference transition by the next continuation weight, divide by the current one, and the ratios telescope. A global preference over whole trajectories has become local driven dynamics, kernel by kernel. Conditioned processes, rare-event driving, Schrödinger bridges and linearly solvable control all live on this branch.
§36↗ · Thm 36.1↗