Local decisions, diffusive influence, and lower bounds for graphical balanced allocation

Abstract: In graphical two-choice allocation, each arriving ball is assigned to one endpoint of a random edge. We study rules whose decision is a monotone function of the two endpoint loads, allowing edge-dependent thresholds and fresh randomization. Such a rule has an exact unit-discrepancy coupling: adding one ball to the initial state produces one tagged discrepancy at every later time. We represent the tag by conditional-expectation projections on the marked edge space and obtain diffusive displacement bounds. A transport-volume inequality then converts slow propagation of influence into lower bounds for the load gap. On the cycle with $n$ vertices, from every initial distribution and at every physical time $t\ge 1/n$, the expected gap is at least a constant times $\min{\sqrt n,t<sup>{1/4}}$, and the gap exceeds this scale with probability at least $1/8$. After exactly $k\ge1$ allocations, the corresponding scale is $\min{\sqrt n,(k/n)<sup>{1/4}}$. No stationarity, symmetry, recurrence, or moment assumption is used. The general inequality also yields a lower bound of order $\sqrt{L/K+\log K}$ on the $L\times K$ discrete torus $C_L\square C_K$. These results separate endpoint-local rules from global-information strategies that achieve polylogarithmic gaps on cycles.