Assertions on the congruence f(x) + g(y) + c ≡ 0 (mod xy) made without proof by Mordell in his paper in Acta Math. 88 (1952) are either proved or disproved.
@article{bwmeta1.element.bwnjournal-article-doi-10_4064-aa167-4-3, author = {A. Schinzel}, title = {On the congruence f(x) + g(y) + c [?] 0 (mod xy) (completion of Mordell's proof)}, journal = {Acta Arithmetica}, volume = {168}, year = {2015}, pages = {347-374}, zbl = {06414117}, language = {en}, url = {http://dml.mathdoc.fr/item/bwmeta1.element.bwnjournal-article-doi-10_4064-aa167-4-3} }
A. Schinzel. On the congruence f(x) + g(y) + c ≡ 0 (mod xy) (completion of Mordell's proof). Acta Arithmetica, Tome 168 (2015) pp. 347-374. http://gdmltest.u-ga.fr/item/bwmeta1.element.bwnjournal-article-doi-10_4064-aa167-4-3/