The following items are available as free downloads subject to the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License http://creativecommons.org/licenses/by-nc-sa/3.0/deed.en_G.
All have been judged to meet the evaluation criteria set by the Editorial Board of the American Institute of Mathematics in connection with the Institute's Open Textbook Initiative. http://www.aimath.org/textbooks/
ELEMENTARY DIFFERENTIAL EQUATIONS http://ramanujan.math.trinity.edu/wtrench/texts/TRENCH_FREE_DIFFEQ_I.PDF, previously published by Brooks/Cole Thomson Learning, 2000.
ELEMENTARY DIFFERENTIAL EQUATIONS WITH BOUNDARY VALUE PROBLEMS, http://ramanujan.math.trinity.edu/wtrench/texts/TRENCH_FREE_DIFFEQ_II.PDF, previously published by Brooks/Cole Thomson Learning, 2000.
STUDENT SOLUTIONS MANUAL FOR ELEMENTARY DIFFERENTIAL EQUATIONS AND ELEMENTARY DIFFERENTIAL EQUATIONS WITH BOUNDARY VALUE PROBLEMS http://ramanujan.math.trinity.edu/wtrench/texts/TRENCH_DE_STUDENT_MANUAL.PDF, previously published by Brooks/Cole Thomson Learning, 2000.
INTRODUCTION TO REAL ANALYSIS http://ramanujan.math.trinity.edu/wtrench/texts/TRENCH_REAL_ANALYSIS.PDF, previously published by Pearson Education, 2003.
Supplements to INTRODUCTION TO REAL ANALYSIS:
FUNCTIONS DEFINED BY IMPROPER INTEGRALS http://ramanujan.math.trinity.edu/wtrench/texts/TRENCH_IMPROPER_FUNCTIONS.PDF, previously published by Harper & Row, 1978.
THE METHOD OF LAGRANGE MULTIPLIERS, http://ramanujan.math.trinity.edu/wtrench/texts/TRENCH_LAGRANGE_METHOD.PDF, previously published by Harper & Row, 1978.
Instructor's solutions manuals are available on request, subject to verification of faculty status.
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William F. Trench
Andrew G. Cowles Distinguished Professor Emeritus
It's also informative to note that the source (e.g. $\LaTeX$ files) is not offered here. As David Farmer might point out, this is a useful example that distinguishes a free text from an open-source text. These texts are free, but are not open-source.
If any of you have experience with these texts or can provide additional information, please leave a comment.