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Topological and algebraic structures on the ring of Fermat reals

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The ring of Fermat reals is an extension of the real field containing nilpotent infinitesimals, and represents an alternative to Synthetic Differential Geometry in classical logic. In the present paper, our first aim is to study this ring by using standard topological and algebraic structures. We present the Fermat topology, generated by a complete pseudo-metric, and the omega topology, generated by a complete metric. The first one is closely related to the differentiation of (non-standard) smooth functions defined on open sets of Fermat reals. The second one is connected to the differentiation of smooth functions defined on infinitesimal sets. Subsequently, we prove that every (proper) ideal is a set of infinitesimals whose order is less than or equal to some real number. Finally, we define and study roots of infinitesimals. A computer implementation as well as an application to infinitesimal Taylor formulas with fractional derivatives are presented.

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Correspondence to Paolo Giordano.

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Supported by an L. Meitner FWF (Austria) grant (M1247-N13).

Supported by FWF research grants Y237 and P20525.

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Giordano, P., Kunzinger, M. Topological and algebraic structures on the ring of Fermat reals. Isr. J. Math. 193, 459–505 (2013).

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