Linear Congruences in several variables with congruence restrictions

In this article, we consider systems of linear congruences in several variables and obtain necessary and sufficient conditions as well as explicit expressions for the number of solutions subject to certain restriction conditions. These results are in terms of Ramanujan sums and generalize the results of Lehmer DNL13 and Bibak et al. BBVRL17. These results have analogues over Fqt where the proofs are similar, once notions such as Ramanujan sums are defined in this set-up. We use the recent description of Ramanujan sums over function fields as developed by Zhiyong Zheng ZZ18. This is discussed in the last section. We illustrate the formulae obtained for the number of solutions through some examples. Over the integers, such problems have a rich history, some of which seem to have been forgotten - a number of papers written on the topic re-prove known results. The present authors also became aware of some of these old articles only while writing the present article and hence, we recall very briefly some of the old work by H. J. S. Smith, Rademacher, Brauer, Butson and Stewart, Ramanathan, McCarthy, and Spilker AB26, BS55, PJM76, HR25, KGR44, HJSS61, JS96.