1. African Institute for Mathematical Sciences AIMS‐Cameroon P.O. Box 608 Limbé Crystal Gardens South West Region Cameroon
2. Departamento de Matemática Aplicada II, E.E. Aeronáutica e do Espazo Universidade de Vigo Campus As Lagoas s/n Ourense 32004 Spain
3. Instituto de Matemáticas Universidade de Santiago de Compostela Santiago de Compostela 15782 Spain
4. Center for Research and Development in Mathematics and Applications (CIDMA), Department of Mathematics University of Aveiro Aveiro 3810‐193 Portugal