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Proposal: Create a tag umbral-calculus.
In this question the asker asks why there is no tag for questions related to the umbral calculus. I am in agreement with Alexander Gruber's comment that the significant number of otherwise unorganized questions on the topic is a good indication that t...
Let $f(x)$ be a function defined on an interval containing $a$, except possibly not at $a$. Let $L\in\mathbb R$. Consider the statements:
$\forall \epsilon>0,~\forall \delta>0,~0<|x−a|<\delta\implies|f(x)−L|<\epsilon$
$\exists \epsilon>0~~s.t.~~\exists\delta>0~~s.t.~~0<|x−a|<\delta\impli...
