Bartsch, T., & Mawhin, J. (1991). The Leray-schauder Degree of S1-equivariant Operators Associated To Autonomous Neutral Equations in Spaces of Periodic-functions. Journal of Differential Equations, 92(1), 90-99. https://doi.org/10.1016/0022-0396(91)90065-H (Original work published 1991)