Kevin Conrad has a more in-depth paper on this at roughly the same maturity level, also motivated by Feynman's story: https://kconrad.math.uconn.edu/blurbs/anal

Kevin Conrad has a more in-depth paper on this at roughly the same maturity level, also motivated by Feynman's story: https://kconrad.math.uconn.edu/blurbs/analysis/diffunderint.pdf https://kconrad.math.uconn.edu/blurbs/analysis/diffunderint....