Four theorems of the author on the subject are given without proofs.
@article{bwmeta1.element.bwnjournal-article-doi-10_4064-bc108-0-19,
author = {A. Schinzel},
title = {On the congruence f(x) + g(y) + c [?] 0 (mod xy)},
journal = {Banach Center Publications},
volume = {108},
year = {2016},
pages = {255-257},
language = {en},
url = {http://dml.mathdoc.fr/item/bwmeta1.element.bwnjournal-article-doi-10_4064-bc108-0-19}
}
A. Schinzel. On the congruence f(x) + g(y) + c ≡ 0 (mod xy). Banach Center Publications, Tome 108 (2016) pp. 255-257. http://gdmltest.u-ga.fr/item/bwmeta1.element.bwnjournal-article-doi-10_4064-bc108-0-19/