Infinitesimal = number? Why infinitesimal can't be a "number"?

Why infinitesimal can't be a "number"?
The answer like all things mathematical depends upon whom one asks.
Read this web page, infinitesimals.
Please read the entire page, particularly the section on teaching.
ElementaryCalculus: An Infinitesimal Approach
InElementary Calculus: An Infinitesimal Approach by Jerome Keisler inchapter 3, there is good proof of this. The chapters and whole bookis a free down-load HERE
 
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