Geodesic
Retracts of structures
Matematičeskie zametki, Tome 7 (1970) no. 6, pp. 687-692 
T. S. Fofanova. Retracts of structures. Matematičeskie zametki, Tome 7 (1970) no. 6, pp. 687-692. http://geodesic.mathdoc.fr/item/MZM_1970_7_6_a3/
@article{MZM_1970_7_6_a3,
     author = {T. S. Fofanova},
     title = {Retracts of structures},
     journal = {Matemati\v{c}eskie zametki},
     pages = {687--692},
     year = {1970},
     volume = {7},
     number = {6},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1970_7_6_a3/}
}
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Voir la notice de l'article provenant de la source Math-Net.Ru

It is proved that the category of all structures does not contain nontrivial injective objects. Necessary and sufficient conditions are derived under which any substructure (ideal, filter) of a structure $L$ is a retract of $L$.