**Non-standard analysis** is a branch of classical mathematics that formulates analysis using a rigorous notion of an infinitesimal number.

Non-standard analysis was introduced in the early 1960s by the mathematician Abraham Robinson. He wrote:

- the idea of infinitely small or
*infinitesimal*quantities seems to appeal naturally to our intuition. At any rate, the use of infinitesimals was widespread during the formative stages of the Differential and Integral Calculus. As for the objection that the distance between two distinct real numbers cannot be infinitely small, Gottfried Wilhelm Leibniz argued that the theory of infinitesimals implies the introduction of ideal numbers which might be infinitely small or infinitely large compared with the real numbers but which were*to possess the same properties as the latter*.

Robinson argued that this law of continuity of Leibniz's is a precursor of the transfer principle. Robinson continued:

- However, neither he nor his disciples and successors were able to give a rational development leading up to a system of this sort. As a result, the theory of infinitesimals gradually fell into disrepute and was replaced eventually by the classical theory of limits.

Robinson continues:

- It is shown in this book that Leibniz's ideas can be fully vindicated and that they lead to a novel and fruitful approach to classical Analysis and to many other branches of mathematics. The key to our method is provided by the detailed analysis of the relation between mathematical languages and mathematical structures which lies at the bottom of contemporary model theory.

In 1973, intuitionist Arend Heyting praised non-standard analysis as "a standard model of important mathematical research".

Read more about Non-standard Analysis: Introduction, Motivation, Approaches To Non-standard Analysis, Invariant Subspace Problem, Other Applications, Critique, Logical Framework, Internal Sets, First Consequences, -saturation

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