New Lower Bounds for Rows of $d$-Disjunct Matrices via Recursive Potentials

In nonadaptive combinatorial group testing, given $n$ items with at most $d$ positives, the goal is to identify them using as few pooled tests as possible. A $t\times n$ binary matrix represents the design, where rows are tests and columns are items. The matrix is $d$-disjunct if no column is contained in the Boolean union of any $d$ others. Let $T(d)$ be the minimum $t$ for which such a matrix exists with $n>t$. Shangguan and Ge proved $T(d)\ge \frac{15+\sqrt{33}}{24}d^2$ by counting private pairs (IEEE Trans. Inf. Theory, 62(12):7518-7521, 2016). In this paper, we strengthen their argument by introducing a column-deletion recurrence in which the light-heavy threshold varies with the recursive state $z=(n-t)/d^2$, rather than remaining fixed. This yields the improved bound $T(d)\ge 0.9283d^2-O(d)$. The analytic core reduces to a first-order ODE, and a self-contained interval-arithmetic certificate verifies that the solution reaches the required contact point.