double precision function dchi (x)
c december 1980 edition.  w. fullerton, bell labs.
c
c evaluate the hyperbolic cosine integral.  when x is negative, the
c principal value is used.
c
      double precision x, dei, de1
      external de1, dei
c
      dchi = 0.5d0 * (dei(x) - de1(x))
c
      return
      end