For any dimension n, let C(n) denote the quantity
C(n):=Γ(n/2+1)πn/2finff^(0)(r/2)nf(0)
where f ranges over integrable continuous functions f:=Rn→R, not identically zero, with f^(ξ)≥0 for all ξ and f(x)≤0 for all ∣x∣≥r for some r>0. Establish upper bounds for C(n) that are as strong as possible.