  It turns out that all instances of the diophantine Frobenius problem for
three coprime a_i have a common geometric structure which is independent of
arithmetic coincidences among the a_i. By exploiting this structure we easily
obtain Johnson's formula for the largest non-representable z, as well as a
formula for the number of such z. A procedure is described which computes these
quantities in O(log(max a_i)) steps.
