Ruscheweyh, Stephan, and C. Luis Salinas. “On the Preservation of Direction-Convexity and the Goodman-Saff Conjecture”. Annales Fennici Mathematici, vol. 14, no. 1, Feb. 1989, pp. 63-73, https://doi.org/10.5186/aasfm.1989.1427.