1. Ambrosetti, A., Malchiodi, A.: Nonlinear Analysis and Semilinear Elliptic Problems. Cambridge Studies in Advanced Mathematics, vol. 104. Cambridge University Press, Cambridge (2006)
2. Baernstein, A.: A Unified Approach to Symmetrization. Partial Differential Equations of Elliptic Type (Cortona, 1992). Sympos. Math. XXXV, pp. 47–91. Cambridge University Press, Cambridge (1994)
3. Brasco, L., Parini, E.: The second eigenvalue of the fractional $$p$$-Laplacian. Adv. Calc. Var. 9(4), 323–355 (2016). https://doi.org/10.1515/acv-2015-0007
4. Cabré, X., Sire, Y.: Nonlinear equations for fractional Laplacians I: regularity, maximum principles, and Hamiltonian estimates. Ann. Inst. H. Poincaré Anal. Non Linéaire 367, 911–941 (2014)
5. Damascelli, L., Pacella, F.: Symmetry results for cooperative elliptic systems via linearization. SIAM J. Math. Anal. 45(3), 1003–1026 (2013)