no. 3
Convexity on unions of sets
Compositio Mathematica, Tome 42 (1980) no. 3, pp. 391-400 
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     author = {Sierksma, Gerard},
     title = {Convexity on unions of sets},
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     pages = {391--400},
     year = {1980},
     publisher = {Sijthoff et Noordhoff International Publishers},
     volume = {42},
     number = {3},
     mrnumber = {607378},
     zbl = {0454.52001},
     language = {en},
     url = {https://www.numdam.org/item/CM_1980__42_3_391_0/}
}
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Sierksma, Gerard. Convexity on unions of sets. Compositio Mathematica, Tome 42 (1980) no. 3, pp. 391-400. https://www.numdam.org/item/CM_1980__42_3_391_0/

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