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We prove that every elementary (∞,1)-topos has a natural number object. We achieve this by defining the loop space of the circle and showing that we can construct a natural number object out of it. Part of the proof involves showing that various definitions of natural number objects (Lawvere, Freyd and Peano) agree with each other in an elementary (∞,1)-topos. As part of this effort we also study the internal object of contractibility in (∞,1)-categories, which is of independent interest. Finally, we discuss various applications of natural number objects. In particular, we use it to define internal sequential colimits in an elementary (∞,1)-topos.
@article{TAC_2021_37_a12, author = {Nima Rasekh}, title = {Every elementary higher topos has a natural number object}, journal = {Theory and applications of categories}, pages = {337--377}, publisher = {mathdoc}, volume = {37}, year = {2021}, language = {en}, url = {http://geodesic.mathdoc.fr/item/TAC_2021_37_a12/} }
Nima Rasekh. Every elementary higher topos has a natural number object. Theory and applications of categories, Tome 37 (2021), pp. 337-377. http://geodesic.mathdoc.fr/item/TAC_2021_37_a12/