Turnwald, Gerhard (1990). "A note on the Ramanujan-Nagell equation". Number-theoretic analysis (Vienna, 1988–89). Lecture Notes in Math. Vol. 1452. Springer, Berlin. pp. 206–207. doi:10.1007/BFb0096992. MR1084649.
Turnwald, Gerhard (1990). "A note on the Ramanujan-Nagell equation". Number-theoretic analysis (Vienna, 1988–89). Lecture Notes in Math. Vol. 1452. Springer, Berlin. pp. 206–207. doi:10.1007/BFb0096992. MR1084649.