#### fresh_42

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2022 Award
We use integration to measure lengths, areas, or volumes. This is a geometrical interpretation, but we want to examine an analytical interpretation that leads us to Integration reverses differentiation. Hence let us start with differentiation.
Weierstraß Definition of Derivatives
##f## is differentiable at ##x## if there is a linear map ##D_{x}f##, such that
\begin{equation*}
\underbrace{D_{x}(f)}_{\text{Derivative}}\cdot \underbrace{v}_{\text{Direction}}=\left(\left. \dfrac{df(t)}{dt}\right|_{t=x}\right)\cdot v=\underbrace{f(x+v)}_{\text{location plus change}}-\underbrace{f(x)}_{\text{location}}-\underbrace{o(v)}_{\text{error}}
\end{equation*}
where the error ##o(v)## increases slower than linear (cp. Landau symbol). The derivative can be the Jacobi-matrix, a gradient, or simply a slope. It is always an array of numbers. If we speak of derivatives as functions, then we mean ##f’\, : \,x\longmapsto D_{x}f.## Integration is the problem to compute ##f## from ##f’## or ##f## from
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Klystron, dextercioby, BWV and 5 others

## Answers and Replies

PF has abundant threads extolling the virtues of the irrational number π, various methods for approximating or calculating its digits after the decimal point, and π's appearance in various formulae.

This Insights article succinctly derives my favorite transcendental number e, its importance as the base of the natural logarithms and the exciting exp and lne inverse functions, and e's appearance in formulae modelling exponential growth. While mildly disappointed at not encountering specific examples of e used in electronic vector theory where I first encountered it, the author amply illustrates the relationship between derivatives and integration.

ohwilleke and Greg Bernhardt