We investigate the descriptive complexity of finite abelian groups. Using Ehrenfeucht-Fraïssé games we find upper and lower bounds on quantifier depth, quantifier alternations, and number of variables of a first-order sentence that distinguishes two finite abelian groups. Our main results are the following. Let G 1 and G 2 be a pair of nonisomorphic finite abelian groups. Then there exists a positive integer m that divides one of the two groups' orders such that the following holds: (1) there exists a first-order sentence ϕ that distinguishes G 1 and G 2 such that ϕ is existential, has quantifier depth O(log m), and has at most 5 variables and (2) if ϕ is a sentence that distinguishes G 1 and G 2 then ϕ must have quantifier depth Ω(log m). These results are applied to (1) get bounds on the first-order distinguishability of dihedral groups, (2) to prove that on the class of finite groups both cyclicity and the closure of a single element are not first-order definable, and (3) give a different proof for the first-order undefinability of simplicity, nilpotency, and the normal closure of a single element on the class of finite groups (their undefinability were shown by Koponen and Luosto in an unpublished paper).
Descriptive Complexity of Finite Abelian Groups 1089are given in Sec. 3. In Sec. 4, EF games are applied to the groups Z p and Z q for prime numbers p and q to find bounds on the quantifier depth of a distinguishing first-order sentence. In Sec. 5, an extended version of EF games (using pebbles) is applied to the same groups to find bounds on the number of variables in a distinguishing first-order sentence. In Sec. 6, the game is applied to groups modulo any number. In Sec. 7 bounds are obtained for any finite abelian groups. In Sec. 8 we use the above bounds to get definability results on the following group-theoretic notions: cyclicity, simplicity, nilpotency, the closure of a single element, and dihedral groups. In Sec. 9 we state some of the open problems to look at.
Ehrenfeucht-Fraïssé GamesAs described above EF games are used as a tool to get upper and/or lower bounds on logical expressibility. An EF -game [6, 4] is played over two structures of the same kind, for example two linear orderings. There are two players: the spoiler denoted by S and the duplicator denoted by D. The game has k rounds, for some non-negative integer k. Intuitively, the goal of S is to show that the two structures can be distinguished in at most k steps, whereas D wants to show that this cannot be done.Definition 2 (Partial isomorphism). Let A and B be two first-order structures with vocabulary τ . Assumeā = a 1 , . . . , a n ∈ A n andb = b 1 , . . . , b n ∈ B n . We say that there is a partial isomorphism fromā ontob if for every m, for every quantifierfree formula ϕ(x 1 , . . . , x m ) over τ , and for every multiset {i 1 , . . . , i m } ⊆ {1, . . . , n} the following holds:If A and B are groups, then partial isomorphism basically means that for every multiset {i 1 , i 2 , i 3 } ⊆ {1, . . . , n}, the following hold:We now des...