David Jao Stephen D. Miller Ramarathnam Venkatesan Ramanujan Graphs and the Random Reducibility of Discrete Log on Isogenous Elliptic Curves http://arxiv.org/abs/math.NT/0411378 2004 CoRR math.NT/0411378 db/journals/corr/corr0411.html#math-NT-0411378 informal publication