A dual linear programming bound for sphere packing in dimension 36

Abstract

We construct an explicit dual-feasible point for the Cohn-Elkies linear program in dimension 36, built from the space of weight-18 modular forms for $Γ_0(24)$ following the method of Cohn and Triantafillou. The certificate shows that the two-point linear programming bound on the sphere packing density in dimension 36 exceeds the density of the best packing currently known -- the Kschischang-Pasupathy packing, of center density $2^{18}/3^{10}$ -- by a factor of at least 32.91. In particular, no Cohn-Elkies auxiliary function can certify the best known packing in dimension 36 as optimal. To our knowledge this is the first such dual bound in any dimension above 32, extending the table of Cohn-Triantafillou ($d=12,16,20,28,32$), Li ($3 \le d \le 13$), and de Courcy-Ireland-Dostert-Viazovska ($d=6$). The certificate is exact: the dual point is a rational vector, coefficient nonnegativity is verified by exact arithmetic up to $n=800$, and eventual positivity of the two relevant $q$-expansions is proved via an explicit Deligne-type tail bound whose constant is certified with outward-rounded interval arithmetic. Two methodological points may be of independent interest: a constraint-generation (cutting-plane) formulation of the exact rational LP, which makes the tail-safe optimum reachable; and a sharpened, lift-aware form of the Deligne bookkeeping constant, $C=\sum_{f,e}|λ_{f,e}|\,e^{-(k-1)/2}$, without which the finite verification in dimension 36 fails marginally.