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Elementary Number Theory and Its Applications is noted for its outstanding exercise sets, including basic exercises, exercises designed to help students explore key concepts, and challenging exercises. Computational exercises and computer projects are also provided. In addition to years of use and professor feedback, the fifth edition of this text has been thoroughly checked to ensure the quality and accuracy of the mathematical content and the exercises. The blending of classical theory with modern applications is a hallmark feature of the text. The Fifth Edition builds on this strength with new examples and exercises, additional applications and increased cryptology coverage. The author devotes a great deal of attention to making this new edition up-to-date, incorporating new results and discoveries in number theory made in the past few years.

Lemma Let U and V be vector spaces, and let b:UxV-->X be a bilinear map from UxV to a vector space X. Suppose that for every bilinear map f defined on UxV there is a unique linear map c defined on X such that f=cb. Then there is an isomorphism i:X-->U@V such that u@v=ib(u,v) for every (u,v) in U@V. We can avoid mentioning u@v if we use the map g:UxV-->U@V. Then the lemma says that g=ib. Briefly, the point of the lemma is that any bilinear map b:UxV-->X satisfying the universal property is isomorphic to the map g:UxV-->U@V in an obvious sense.

This coincides with the usual definition of the rank of a matrix. (Exercise: prove this.)

泛函分析答案(张恭庆)