We give a simple criterion for the solvability in integers of the negative Pell equation $x^2 - dy^2 = -1$ by means of primitive Pythagorean triples. The method of proof allows us to also solve the problem of determinig the fundamental units of negative norm in real quadratic fields.
Aleksander Grytczk. Florian Luca. Marek Wójtowicz. "The negative Pell equation and Pythagorean triples." Proc. Japan Acad. Ser. A Math. Sci. 76 (6) 91 - 94, June 2000. https://doi.org/10.3792/pjaa.76.91