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one of the most beautiful techniques

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a quaternion version of Euler's formula

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Hyperreal Numbers: An Introduction to Infinitesimals and Nonstandard Analysis

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What Lies Between a Function and Its Derivative? | Fractional Calculus

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What is Jacobian? | The right way of thinking derivatives and integrals

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All the Numbers - Numberphile

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5 Levels Of “No Answer" (when should we use what?)

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The most interesting differential equation you have seen.