Worst Cases for the Exponential Function in the IEEE 754r decimal64 Format

  • Authors:
  • Vincent Lefèvre;Damien Stehlé;Paul Zimmermann

  • Affiliations:
  • INRIA/ÉNS Lyon, Université de Lyon/LIP, Lyon Cedex 07, France F-69364;CNRS/ÉNS Lyon, Université de Lyon/LIP/INRIA Arenaire, Lyon Cedex 07, France F-69364;LORIA/INRIA Lorraine, Bâtiment A, Technopôle de Nancy-Brabois, , Villers-lès-Nancy Cedex, France F-54602

  • Venue:
  • Reliable Implementation of Real Number Algorithms: Theory and Practice
  • Year:
  • 2008

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Abstract

We searched for the worst cases for correct rounding of the exponential function in the IEEE 754r decimal64 format, and computed all the bad cases whose distance from a breakpoint (for all rounding modes) is less than 10驴 15ulp, and we give the worst ones. In particular, the worst case for |x| 驴 3 ×10驴 11is $\exp(9.407822313572878 \times 10^{-2}) = 1.098645682066338\,5\,0000000000000000\,278\ldots$. This work can be extended to other elementary functions in the decimal64 format and allows the design of reasonably fast routines that will evaluate these functions with correct rounding, at least in some domains.