On a cardinal group invariant related to decompositions of Abelian groups

For each Abelian groupG, a cardinal invariant χ(G) is introduced and its properties are studied. In the special caseG = ℤ ⁿ , the cardinalχ(ℤ ⁿ ) is equal to the minimal cardinality of an essential subset of ℤ ⁿ , i.e., a of a subsetA ⊂ ℤ ⁿ such that, for any coloring of the group ℤ ⁿ inn colors, there exists an infinite one-color subset that is symmetric with respect to some pointα ofA. The estimaten( ^{n +} l)/2 ≤χ(ℤ ⁿ ) < 2n is proved for alln and the relationχ(ℤ ⁿ ) =n(n + 1)/2 forn ≤ 3. The structure of essential subsets of cardinalityχ(ℤ ⁿ ) in ℤ ⁿ is completely described forn ≤ 3. Translated fromMatematicheskie Zametki, Vol. 64, No. 3, pp. 341–350, September, 1998.