results so that it is also useful as a reference book. It is designed for use at both undergraduate and graduate level. This is possible because of its unique "two-tier" structure: the text speaks directly to the undergraduate in extremly clear verbal and graphic terms; the appendices, which form the second tier, are addressed to the graduate student and the researcher and are self-contained treatments in mathematical terms of the topics examined in the text. The ample and balanced treatment of the various approaches and the clarity of exposition ensure that the reader gains a thorough grasp of theories, facts and policies.

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