Question: What is the most commonly used Calculus textbook in American universities?

Math 55

Math 55 is a two-semester long first-year undergraduate mathematics course at Harvard University, founded by Lynn Loomis and Shlomo Sternberg. The official titles of the course are Honors Abstract Algebra (Math 55a) and Honors Real and Complex Analysis (Math 55b). Previously, the official title was Honors Advanced Calculus and Linear Algebra.
Description
The Harvard University Department of Mathematics describes Math 55 as "probably the most difficult undergraduate math class in the country". [1] Formerly, students would begin the year in Math 25 (which was created in 1983 as a lower-level Math 55) and, after three weeks of point-set topology and special topics (for instance, in 1994, p-adic analysis was taught by Wilfried Schmid), students would take a quiz. As of 2012, students may choose to enroll in either Math 25 or Math 55 but are advised to "shop" both courses and have five weeks to decide on one.[2] Depending on the professor teaching the class, the diagnostic exam may still be given after three weeks to help students with their decision.
In 1994, 89 students took the test given after three weeks: students scoring more than 50% on the quiz could enroll in Wilfried Schmid's Math 55 (15 students), students scoring between 10 and 50% could stay in Benedict Gross's Math 25 (55 students), and students scoring less than 10% were advised to enroll in a course such as Math 21, multivariate calculus (19 students).[3] A take-home final ends the class.[4]
Historical retention rate
In 1970, Math 55 covered almost four years worth of department coursework in two semesters, and subsequently, it drew only the most diligent of undergraduates. Of the 75 students who enrolled in the 1970 offering, by course end, only 20 remained due to the advanced nature of the material and time-constraints under which students were given to work.[5] David Harbater, a University of Pennsylvania mathematics professor/researcher, and survivor of the 1974 Math 55 section at Cambridge, recalled of his experience, "Seventy [students] started it, 20 finished it, and only 10 understood it." Scott D. Kominers, familiar with the stated attrition rates for the course, decided to keep an informal log of his journey through the 2009 section: "...we had 51 students the first day, 31 students the second day, 24 for the next four days, 23 for two more weeks, and then 21 for the rest of the first semester after the fifth Monday." (The beginning of the fifth week being the drop-deadline for students to decide whether to remain in Math 55, or transfer to Math 25 (Linear Algebra and Real Analysis I & II)).[6]
Course content
Through 2006,[7] the instructor had broad latitude in choosing the content of the course. Though Math 55 bore the official title "Honors Advanced Calculus and Linear Algebra", advanced topics in complex analysis, point set topology, group theory, and/or differential geometry could be covered in depth at the discretion of the instructor, in addition to single and multivariable real analysis and abstract linear algebra. In 1970, for example, students studied the differential geometry of Banach manifolds in the second semester of Math 55.[5] In contrast, Math 25, entitled "Honors Multivariable Calculus and Linear Algebra", tended to be more narrowly focused, usually covering real analysis, together with the relevant theory of metric spaces and (multi)linear maps. These topics typically culminated in the proof of the generalized Stokes' theorem, though, time permitting, other relevant topics (e.g., category theory, de Rham cohomology) might also be covered.[8]  Although both courses presented calculus from a rigorous point of view and emphasized theory and proof writing, Math 55 was generally faster paced, more abstract, and demanded a higher level of mathematical sophistication.  
Loomis and Sternberg's textbook Advanced Calculus,[9] an abstract treatment of calculus in the setting of normed vector spaces and on differentiable manifolds, was tailored to the authors' Math 55 syllabus and served for many years as an assigned textbook. Over the years, instructors for Math 55[10] and Math 25[8] have also selected Rudin's Principles of Mathematical Analysis,[11] Spivak's Calculus on Manifolds,[12] Axler's Linear Algebra Done Right,[13] and Halmos's Finite-Dimensional Vector Spaces[14] as textbooks or references.
From 2007 onwards, the scope of the course (along with that of Math 25) was changed to more strictly cover the contents of four semester-long courses in two semesters: Math 25a (linear algebra) and Math 122 (group theory) in Math 55a; and Math 25b (calculus, real analysis) and Math 113 (complex analysis) in Math 55b. The name was also changed to "Honors Abstract Algebra" (Math 55a) and "Honors Real and Complex Analysis" (Math 55b).  Fluency in formulating and writing mathematical proofs is listed as a course prerequisite for Math 55, while such experience is considered "helpful" but not required for Math 25.[1] In practice, students of Math 55 have usually had extensive experience in proof writing and abstract mathematics, and many are winners of prestigious national or international mathematical olympiads (e.g., USAMO or IMO), while typical students of Math 25 have also had previous exposure to proof writing through mathematical contests or university level proof-based courses.
Notable alumni
Problem sets are expected to take from 24 to 60 hours per week to complete,[1] although some claim that it is closer to 20 hours.[15] Of those students who could handle the workload, some became math or physics professors,[5] including members of the Harvard Math Department such as Benedict Gross and Joe Harris; also, Harvard physics professor Lisa Randall '84[16] and Harvard economics professor Andrei Shleifer '82.[17] Although a 2006 Harvard Crimson article alleged that only 17 women completed the class between 1990 and 2006,[6] in fact 39 women completed 55a (the first of the two semesters), and 26 completed 55b.[18] Math 25 has more women: in 1994–95, Math 55 had no women, while Math 25 had about 10 women in the 55 person course.[3]
There are also Math 55 alumni who went on to be professors in other universities. These include Fields-medalist Manjul Bhargava, who is now a professor at Princeton University, as well as Kiran Kedlaya, now at the University of California, San Diego.
In addition to these professors, past students of Math 55 include Bill Gates[19], Richard Stallman.[5] and Simpsons executive producer Al Jean.[20]
Demographics of students taking this course over the years has been used to study causes of gender and race differences in the fields of mathematics and technology.[21]
Historical instances of Math 55

Fictional references
Math 55, along with several other high-level mathematics courses, were brought up by Dr. Spencer Reid in a 2015 episode of Criminal Minds entitled "Mr. Scratch."[33]




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