NewtonInvert[n, digits=getPrecision[]] :=
{
   origPrec = getPrecision[]
   setPrecision[20]
   x = 1./n
   log2 = log[2]
   iters = ceil[ log[(digits+1)/(log[17]/log2)] / log2 ]
   goodDigits = 20
   err = x * 10^-(digits)
   println["err is $err"]
   do
   {
      prec = min[digits+2, 2 * goodDigits]
      setPrecision[prec]
      oldX = x
      x = x (2 - n x)
//      println["x is $x"]
//      setPrecision[5]
      diff = abs[x-oldX]
//      println["diff is $diff"]
      if (diff == 0.0)
         goodDigits = digits + 2
      else
         goodDigits = ceil[(-approxLog2[diff] + 4)/ 3.321928094]
      println["Iter $i, goodDigits is " + goodDigits /* + "x is $x"*/]
   } while (diff > err)
   println["bailing.  diff was $diff"]
   return x
}

/*
start = now[]
n = NewtonInvert[640320,100000]
end = now[]
println["Time for inversion: " + (end-start)]

start = now[]
println[n]
end = now[]
println["Time for formatting: " + (end-start)]
*/