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DCOSH

       DOUBLE PRECISION FUNCTION  DCOSH(X)
 C$    (Hyperbolic Cosine)
 C$    Return  the  hyperbolic  cosine  of  the  DOUBLE  PRECISION
 C$    argument X.
 C$
 C$    The host exponential function is used for the  computation,
 C$    with precautions taken  to prevent  premature overflow  for
 C$    large abs(X).
 C$
 C$    The  ELEFUNT  tests  of  W.J.  Cody,  Jr.   and  W.  Waite,
 C$    "Software   Manual   for    the   Elementary    Functions",
 C$    Prentice-Hall, Englewood Cliffs, NJ,  USA (1980) have  been
 C$    made on the DEC-20/60, with the following results:
 C$
 C$    abs(x) in 0 .. 1/2   - maximum relative error: 1.00 bits lost
 C$                         - RMS relative error:     0.00 bits lost
 C$    abs(x) in 3 .. 88.03 - maximum relative error: 2.43 bits lost
 C$                         - RMS relative error:     0.86 bits lost
 C$    (25-JUN-83)