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% \MRhref is called by the amsart/book/proc definition of \MR.
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\begin{thebibliography}{Apo69}

\bibitem[Apo69]{apostol}
Tom~M. Apostol, \emph{Calculus. {V}ol. {II}: {M}ulti-variable calculus and
  linear algebra, with applications to differential equations and probability},
  Second edition, Blaisdell Publishing Co. Ginn and Co., Waltham,
  Mass.-Toronto, Ont.-London, 1969. \MR{0248290}

\end{thebibliography}