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For advanced students, the mathematical theory has been peer-reviewed and published, available without cost at https://www.mdpi.com/2227-7390/11/1/68
Calculus, as well-known, was invented by Hindu mathematicians (led by Mādhava of Sangamagrāma) 250 years before Newton and Leibniz, and they did not use the fictions of infinitesimals, irrational numbers, or imaginary numbers.