6 PETER H. KRAUSS AND DAVID M. CLARK 1. PRELIMINARIES In this section we shall first make a few remarks on notation and ter- minology, and then we shall review some well-known facts for the purpose of reference. Given is a similarity type determined by a set Rl of relation symbols and a set Op of operation symbols. A structure Oh has universe A = I 011, and for each n-ary relation symbol R € Rl has an n-ary relation R , and for each n-ary operation symbol f € Op has an n-ary operation f . OL is called an algebra if Rl = 0 and 01 is called a relational struc- ture if Op = 0. If Wi is a class of structures then IVKI, SWt and H'Hx denote the classes of isomorphic images, substructures and homomorphic images of members of VYL respectively, and Finn, PVH, P inn and P yn denote the classes of direct products, subdirect products, reduced direct products, and ultraproducts of subsets of YV\ respectively. We allow the direct product of the empty set of structures. Next we introduce the (finitary) first-order language of the given similarity type. Then AW?, 0»l and EWX denote the atomical, universal and elementary classes generated by W\ respectively. For a class VU% of algebras, AVH is called the variety generated by Wi and is denoted by VIM. For any structure Ol , D is the trivial congruence on #L and ± is the universal congruence on Ol . Let *r c n0l. | i € I. For each F s I, TT_ is the projection of 1 r n(Jl. | i € I onto T70l. I i € ¥, and 9 and 9** are the projection congruences on T\01. | i € I and 5& - induced by TT_, respectively. 06- is 1 r called non-trivially subdirect if c& is subdirect and for no i € I is 7T.: ^.- Ol . an isomorphism. If x, y € B then 9 (x, y) denotes the smallest congruence x o n & such that xxy. A congruence x o n "6- is called principal if there exist x, y € B such that x = 0 (x y)» If a is an ordinal and x € (T7A. I i € I)a then TT_(X) € (ffA. j i € F ) a is de- 1 r 1 fined by TTF(X)(€) = TTF(X(C): If cp is a formula and x € A^ then Ol \= cp[x] means that x satisfies tp in OL , and we adopt the usual conventions to make satisfaction of for- mulas by finite sequences well-behaved. If x € (flA. ) n then we define

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