# A Shorter Model Theory by Wilfrid Hodges

By Wilfrid Hodges

This can be an updated textbook of version conception taking the reader from first definitions to Morley's theorem and the straight forward components of balance thought. along with ordinary effects akin to the compactness and omitting kinds theorems, it additionally describes a variety of hyperlinks with algebra, together with the Skolem-Tarski approach to quantifier removing, version completeness, automorphism teams and omega-categoricity, ultraproducts, O-minimality and constructions of finite Morley rank. the fabric on back-and-forth equivalences, interpretations and zero-one legislation can function an advent to purposes of version concept in machine technological know-how. each one bankruptcy finishes with a quick remark at the literature and proposals for extra interpreting. This ebook will profit graduate scholars with an curiosity in version conception.

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10), then we obtain the relation (16) For n = 1, 2, 3, relation (16) yields the relations (17) (18) (19) 37 Properties of Combinations Using Formulas (17)—(19), we can easily find the sum of the squares and the sum of the cubes of the natural numbers from 1 to m. Formula (18) can be rewritten as follows: l 2 + 2 2 + ··· + m2 + 1 + 2 + ··· + m = — ^ —. By Formula (17), 1 + 2 + - + m = m(m + l)/2. Therefore, l 2 + 22 + - + r o 2 ! _ m(m + l)(*ft + 2) _ m(m + 1) 3 _ m(m + l)(2m + 1) ~~ 6 (20) Similar manipulation of Formula (19) yields the relation l 3 + 2 3 + ··· + nr = — — - — .

A combination is put into the &th class if and only if it contains exactly k copies of the letter a. , x. Thus the number of combinations in the &th class is equal to the number of (m — /^-combinations with repetitions of elements of n types, that is, C'^~/^_k_1 . It follows that the number of all the combinations is On the other hand, we know that this number is C™+m . This proves that (15) If in Eq. (15) we change n to n + 1 and m to m — 1 and make use of Eq. (10), then we obtain the relation (16) For n = 1, 2, 3, relation (16) yields the relations (17) (18) (19) 37 Properties of Combinations Using Formulas (17)—(19), we can easily find the sum of the squares and the sum of the cubes of the natural numbers from 1 to m.

000 Derangements* The techniques of the previous section enable us to solve the following problem: Find the number Dn of permutations ofn elements in which no element stays in its original position. The solution is given by the formula (3) A reader familiar with the theory of series will note that the expression in brackets is a partial sum of the series for e~x. It is convenient to extend Formula (3) to the case n = 0. The natural definition isD0 = 1. The number Dnr of permutations in which just r elements remain in their original positions and the remaining n — r elements change their positions is given by (4) * This section can be omitted in a first reading of the book.