Geodesic
The topological center of the semigroup of free ultrafilters
Matematičeskie zametki, Tome 63 (1998) no. 3, pp. 437-441 
I. V. Protasov. The topological center of the semigroup of free ultrafilters. Matematičeskie zametki, Tome 63 (1998) no. 3, pp. 437-441. http://geodesic.mathdoc.fr/item/MZM_1998_63_3_a14/
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     title = {The topological center of the semigroup of free ultrafilters},
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Voir la notice de l'article provenant de la source Math-Net.Ru

For any group $G$, it is proved that the topological center of the semigroup of free ultrafilters on the group $G$ is empty.

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[4] Blass A., Hindman N., “On strongly summable and union ultrafilters”, Trans. Amer. Math. Soc., 304:1 (1987), 83–99 | DOI | MR | Zbl