URL: https://www.researchgate.net/profile/Abdelouahab-Chikh-Salah-2/publication/323906147/figure/fig3/AS:606506852552704@1521613910425/figure-fig3_Q320.jpg
URL2: https://www.researchgate.net/figure/The-standard-contact-structure-in-R-3_fig1_323906147