Pavol Jan Zlatos (2021). “Gordon's conjectures 1 and 2: Pontryagin-van Kampen duality in the hyperfinite setting”. Journal of Logic and Analysis13 (1). doi:10.4115/jla.2021.13.1.
Pavol Jan Zlatos (2021). “Gordon's Conjecture 3: Fourier transforms in the hyperfinite setting”. Journal of Logic and Analysis13 (7). doi:10.4115/jla.2021.13.7.
Edward Nelson: Radically Elementary Probability Theory, Princeton University Press, 1987, full text
Edward Nelson: Internal Set Theory: A New Approach to Nonstandard Analysis, Bulletin of the American Mathematical Society, Vol. 83, Number 6, November 1977. A chapter on Internal Set Theory is available at https://web.math.princeton.edu/~nelson/books/1.pdf