1. School of Mathematical Sciences, University of Nottingham, University Park, Nottingham NG7 2RD, UK
2. Instituto de Estructuras and Departamento de Mecánica, Universidad Nacional de Tucumán, Av. Independencia 1800, San Miguel de Tucumán, Argentina
3. Department of Mathematics, Swansea University, Singleton Park, Swansea SA2 8PP, UK