Flat-top 5-term cosine-sum window on [-1/2 1/2] (dips negative near the edges)
FlatTopWindow[x]
Plot[FlatTopWindow[x], {x, -1, 1}, PlotRange -> All]
→ -Graphics-Plot3D[FlatTopWindow[x]FlatTopWindow[y], {x, -1, 1}, {y, -1, 1}, PlotRange -> All]
→ -Graphics3D-FlatTopWindow[0.1]
→ 0.6068721495762119