uarantee the function has a limit. The theorem
  requires every approach be consistent, which includes parabolic
  approaches, spiraling approaches, and more. Sometimes the
  straight-line paths happen to be consistent with each other, but
  a different path gives a different limit. Give some thought to
  this in the optional challenge problem below.</p>

  <div class="problem-nonum optional" label="Challenge">
    <div class="instructor marginpar"><a href=
    "https://sagecell.sagemath.org/?z=eJyVVNFq2zAU