Wikipedia can be a useful starting place for topics in math. The quality varies, so consult references to validate. References may also lead to additional valuable information.
Undergraduate-Level Book Series
Student Mathematical Library (STML)
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Publishers' Note:"The AMS undergraduate series, the Student Mathematical Library, is for books that will spark students' interests in modern mathematics and increase their appreciation for research. Books published in the series emphasize original topics and approaches. The step from mathematical coursework to mathematical research is one of the most important developments in a mathematician's career. To make the transition successfully, the student must be motivated and interested in doing mathematics rather than merely learning it. They are suitable for honors courses, upper-division seminars, reading courses, or self-study."
MAA Textbooks
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Publisher's Note: "MAA Textbooks cover all levels of the undergraduate curriculum, with a focus on textbooks for upper division students. They are written by college and university faculty, and are carefully reviewed by an editorial board of teaching faculty in order to ensure superior exposition. The editorial board is especially interested in innovative manuscripts."
Undergraduate Texts in Mathematics (Springer)
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Publisher Description: "Undergraduate Texts in Mathematics are generally aimed at third- and fourth-year undergraduate mathematics students at North American universities. These texts strive to provide students and teachers with new perspectives and novel approaches. The books include motivation that guides the reader to an appreciation of interrelations among different aspects of the subject. They feature examples that illustrate key concepts as well as exercises that strengthen understanding."
Dolciani Mathematical Expositions (MAA)
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Publisher's Note: Aimed at a broad audience, books in this series present mathematical elegance and ingenuity in topics such as: number theory, geometry, combinatorics, and the history of mathematics. Several excellent problem books are included in this series. Assumed levels of background range up to that of an undergraduate mathematics major.
Undergraduate-Level Journals
American Mathematical Monthly
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This is an expository journal of collegiate mathematics published by the MAA. Challenging problems and solutions section in every issue. We have print and online issues.
College Mathematics Journal
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From the publisher: "The journal features interviews and articles on history, philosophy, problem solving, applications, computer-related mathematics, Student Research Projects, Classroom Capsules, Media Highlights, book reviews, and problems and solutions."
Notices of the American Mathematical Society
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Lots of information useful to the mathematics major - from accessible articles to state of math education to doctoral program information. Also information on upcoming conferences and career news on this site.
SIAM Review
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Mathematics Magazine
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From the publisher: "Mathematics Magazine offers lively, readable, and appealing exposition on a wide range of mathematical topics."
Bulletin of the American Mathematical Society
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Stated by the AMS: "The Bulletin publishes expository articles on contemporary mathematical research, written in a way that gives insight to mathematicians who may not be experts in the particular topic. The Bulletin also publishes reviews of selected books in mathematics and short articles in the Mathematical Perspectives..."
Mathematical Intelligencer
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The Mathematical Intelligencer publishes articles about math, mathematicians, humor, puzzles, and include expository articles of interest to students of math.
CHANCE
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A non-technical magazine that provides applications for statistical practice.