J C. Alexander, A Primer on Connectivity, Lecture Notes in Math., vol. 886, Springer–Verlag, Berlin, New York, 1981, pp. 455–483.

J. Appell, E. De Pascale and A. Vignoli, Nonlinear Spectral Theory, de Gruyter, Berlin, 2004.

P. Benevieri, A. Calamai, M. Furi and M.P. Pera, On the persistence of the eigenvalues of a perturbed Fredholm operator of index zero under nonsmooth perturbations, Z. Anal. Anwend. 36 (2017), no. 1, 99–128.

P. Benevieri, A. Calamai, M. Furi and M.P. Pera, Global continuation of the eigenvalues of a perturbed linear operator, Ann. Mat. Pura Appl. 197 (2018), no. 4, 1131–1149.

P. Benevieri, A. Calamai, M. Furi and M.P. Pera, Global continuation in Euclidean spaces of the perturbed unit eigenvectors corresponding to a simple eigenvalue, Topol. Methods in Nonlinear Anal. 55 (2020), no. 1, 169–184.

P. Benevieri, A. Calamai, M. Furi and M.P. Pera, Global persistence of the unit eigenvectors of perturbed eigenvalue problems in Hilbert spaces, Z. Anal. Anwend., 39 (2020), no. 4, 475–497.

R. Chiappinelli, Isolated connected eigenvalues in nonlinear spectral theory, Nonlinear Funct. Anal. Appl. 8 (2003), no. 4, 557–579.

R. Chiappinelli, Approximation and convergence rate of nonlinear eigenvalues: Lipschitz perturbations of a bounded self-adjoint operator, J. Math. Anal. Appl. 455 (2017), no. 2, 1720–1732.

R. Chiappinelli, What do you mean by “nonlinear eigenvalue problems”?, Axioms 7 (2018), paper no. 39, 30 pp.

R. Chiappinelli, M. Furi and M.P. Pera, Normalized eigenvectors of a perturbed linear operator via general bifurcation, Glasg. Math. J. 50 (2008), no. 2, 303–318.

R. Chiappinelli, M. Furi and M.P. Pera, Topological persistence of the normalized eigenvectors of a perturbed self-adjoint operator, Appl. Math. Lett. 23 (2010), no. 2, 193–197.

R. Chiappinelli, M. Furi and M.P. Pera, Persistence of the normalized eigenvectors of a perturbed operator in the variational case, Glasg. Math. J. 55 (2013), no. 3, 629–638.

R. Chiappinelli, M. Furi and M.P. Pera, Topological persistence of the unit eigenvectors of a perturbed Fredholm operator of index zero, Z. Anal. Anwend. 33 (2014), no. 3, 347–367.

M. Furi and M.P. Pera, A continuation principle for periodic solutions of forced motion equations on manifolds and applications to bifurcation theory, Pacific J. Math. 160 (1993), no. 3, 219–244.

M.W. Hirsch, Differential Topology, Graduate Texts in Math., vol. 33, Springer–Verlag, Berlin, 1976.

C. Kuratowski, Topology, vol. 2, Academic Press, New York, 1968.

J.M. Milnor, Topology from the Differentiable Viewpoint, Univ. Press of Virginia, Charlottesville, 1965.

L. Nirenberg, Topics in Nonlinear Functional Analysis, Courant Institute of Mathematical Sciences, New York, 1974.

E. Outerelo and J.M. Ruiz, Mapping degree theory, Graduate Studies in Math., Vol. 108, American Mathematical Soc., Providence, RI; Real Soc. Matemática Española, Madrid, 2009.