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[2] HARDY G. H., WRIGHT E. M.: An Introduction to the Theory of Numbers. Second edition. Clarendon Press, Oxford, 1945.

[3] LUBELSKI S.: Zur Theorie der höheren Kongruenzen. J. reine und angew. Math. 162, 1930, 63-68.

[4] SCHWARZ Š.: The role of semigroups in the elementary theory of numbers. Math. Slovaca 31, 1981, 369-395. MR 0637966 | Zbl 0474.10002