mathworld.wolfram.com
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MathWorld is an online mathematics encyclopedia by Wolfram Research that provides extensive articles, definitions, and examples on topics from algebra and calculus to geometry and number theory, including detailed treatments of functions with formulas and diagrams.
More resources on Functions
Introduction to Mathematical Thinking
Learn how to think the way mathematicians do – a powerful cognitive process developed over thousands of years. Mathematical thinking is not the same as doing mathematics – at least not as mathematics is typically presented in our school system. School math typically focuses on learning procedures to solve highly stereotyped problems. Professional mathematicians think a certain way to solve real problems, problems that can arise from the everyday world, or from science, or from within mathematics itself. The key to success in school math is to learn to think inside-the-box. In contrast, a key feature of mathematical thinking is thinking outside-the-box – a valuable ability in today’s world. This course helps to develop that crucial way of thinking.
ocw.mit.edu
MIT OpenCourseWare (OCW) provides free, openly licensed course materials from MIT across a wide range of subjects, including mathematics, with lecture notes, assignments, and video lectures for self-paced learning.
khanacademy.org
Khan Academy is a free online learning platform offering self-paced courses with instructional videos and interactive practice, including a Functions unit on function notation, graphing, domain and range, and real-world applications.