==> analysis/integral.p <==
If f is integrable on (0,inf) and differentiable at 0, and a > 0, and:

      inf           f(x)
   Int        ----------------  dx is defined
       0             x

show:

      inf     ( f(x) - f(ax) )  
   Int        ----------------  dx   = f(0) ln(a)
       0             x