In which categories are first-order axiomatizable hulls characterizable by ultraproducts ?
Cahiers de topologie et géométrie différentielle, Tome 24 (1983) no. 2, pp. 215-222. 
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     author = {Hien, Bui Huy and Sain, I.},
     title = {In which categories are first-order axiomatizable hulls characterizable by ultraproducts ?},
     journal = {Cahiers de topologie et g\'eom\'etrie diff\'erentielle},
     pages = {215--222},
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     volume = {24},
     number = {2},
     year = {1983},
     mrnumber = {710042},
     zbl = {0519.18003},
     language = {en},
     url = {http://archive.numdam.org/item/CTGDC_1983__24_2_215_0/}
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Hien, Bui Huy; Sain, I. In which categories are first-order axiomatizable hulls characterizable by ultraproducts ?. Cahiers de topologie et géométrie différentielle, Tome 24 (1983) no. 2, pp. 215-222. http://archive.numdam.org/item/CTGDC_1983__24_2_215_0/

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