## Abstract

In this paper we investigate the existence of solutions to the following Schrödinger system in the critical case

Here, \(\Omega \subset {\mathbb {R}}^{4}\) is a smooth bounded domain, \(d\ge 2\), \(-\lambda _{1}(\Omega )<\lambda _{i}<0\) and \(\beta _{ii}>0\) for every *i*, \(\beta _{ij}=\beta _{ji}\) for \(i\ne j\), where \(\lambda _{1}(\Omega )\) is the first eigenvalue of \(-\Delta \) with Dirichlet boundary conditions. Under the assumption that the components are divided into *m* groups, and that \(\beta _{ij}\ge 0\) (cooperation) whenever components *i* and *j* belong to the same group, while \(\beta _{ij}<0\) or \(\beta _{ij}\) is positive and small (competition or weak cooperation) for components *i* and *j* belonging to different groups, we establish the existence of nonnegative solutions with *m* nontrivial components, as well as classification results. Moreover, under additional assumptions on \(\beta _{ij}\), we establish existence of least energy positive solutions in the case of mixed cooperation and competition. The proof is done by induction on the number of groups, and requires new estimates comparing energy levels of the system with those of appropriate sub-systems. In the case \(\Omega ={\mathbb {R}}^4\) and \(\lambda _1=\cdots =\lambda _m=0\), we present new nonexistence results. This paper can be seen as the counterpart of Soave and Tavares (J Differ Equ 261:505–537, 2016) in the critical case, while extending and improving some results from Chen and Zou (Arch Ration Mech Anal 205:515–551, 2012) and Guo et al. (Nonlinearity 31:314–339, 2018).

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## Acknowledgements

H. Tavares is partially supported by the Portuguese government through FCT/Portugal - Fundação para a Ciência e a Tecnologia, I.P. under the Project PTDC/MAT-PUR/28686/2017 and through the Grant UID/MAT/04561/2013. S. You would like to thank the China Scholarship Council of China (No.201806180084) for financial support during the period of his overseas study and to express his gratitude to the Department of Mathematics, Faculty of Sciences of the University of Lisbon for its kind hospitality.

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Tavares, H., You, S. Existence of least energy positive solutions to Schrödinger systems with mixed competition and cooperation terms: the critical case.
*Calc. Var.* **59**, 26 (2020). https://doi.org/10.1007/s00526-019-1694-x

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DOI: https://doi.org/10.1007/s00526-019-1694-x