*Problem #125*

When students first learn that *dx*/*dy* is the reciprocal of *dy*/*dx*,
many erroneously jump to the conclusion that
*d*^{2}x/*dy*^{2} is the reciprocal of
*d*^{2}y/*dx*^{2}. This month's problem is to find a family of functions
*y* = *f*(*x*) for which this does hold.