What exactly is a Limit ?

Math Online Tom Circle

$latex displaystylelim_{xto a}f(x) = L
iff$
$latex forall varepsilon >0, exists delta >0 $ such that
$latex boxed{0<|x-a|<delta}
implies |f(x)-L|< varepsilon $

The above scary ‘epsilon-delta’ definition of “Limit” by the French mathematician Cauchy in 19th century is the standard rigorous definition in today’s Analysis textbooks.

It was not taught in my Cambridge GCE A-Level Pure Math in 1970s (still true today), but every French Baccalaureate Math student (Terminale,  equivalent to JC 2 or Pre-U 2) knows it by heart. A Cornel University Math Dean recalled how he was told by his high-school teacher to memorise it — even though he did not fully understand — the “epsilon-delta” definition by “chanting”:

“for all epsilon, there is a delta ….”

(French: Quelque soit epsilon, il existe un delta …)

In this video, I am glad someone like Prof N. Wildberger recognised its “flaws”  albeit rigorous, by suggesting another more intuitive…

View original post 64 more words

Author: tomcircle

Math amateur

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