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It is shown that recently measured cross sections for double ionization ofnegative ions ($H^-, O^-$, and $C^-$) possess a universal shape when plotted insuitable dimensionless units. The shape can be represented with a simpleanalytical function, following the same principles as it has been done inestablishing a universal shape function for single ionization [Rost and Pattard1997 Phys. Rev. A {\bf 55} R5]. Thereby, it is demonstrated that direct doubleionization dominates the cross section for the targets considered.Comment: 4 pages, 3 figure

Universal shape function for the double-ionization cross section of negative ions by electron impact