Calculus is another popular mathematics course, so OER adopters will likewise find a large number of excellent calculus textbooks. Important to keep in mind for this category is the fact that most colleges teach calculus as a multisemester course, desiring to use the same textbook throughout all semesters. Thus, a high quality OER textbook should be able to cover all three semesters of calculus. APEX Calculus by Gregory Hartman covers the usual materials of three-semester calculus with the strongest features. Two other texts that distinguish themselves from the many OER texts in this category are Active Calculus, by Matthew Boelkins, David Austin, and Steven Schlicker, and Community Calculus by David Guichard.