subroutine ttest ( t, df, ans, kerr )

c*********************************************************************72
c
cc ttest
c
      implicit none

      real ans
      real d1
      real d2
      integer df
      real f1
      real f2
      integer i
      integer kerr
      integer n
      real t
      real t1
      real t2

      save d1

      data d1 / 0.63661977236758134e+00 /
c
c  0.63661977236758134... = 2 / pi
c
      kerr = 0
c
c  error return if df not positive.
c
      if ( df .le. 0 ) then
        kerr = 1
        ans = 0.0e+00
        return
      end if
c
c  begin computation of series.
c
      t2 = t * t / float ( df )
      t1 = sqrt ( t2 )
      t2 = 1.0e+00 / ( 1.0e+00 + t2 )

      if ( ( df / 2 ) * 2 .eq. df ) go to 5
c
c  df is an odd integer.
c
      ans = 1.0e+00 - d1 * atan ( t1 )

      if ( df .eq. 1 ) go to 4

      d2 = d1 * t1 * t2
      ans = ans - d2

      if ( df .eq. 3 ) go to 4

      f1 = 0.0e+00
2     n = ( df - 2 ) / 2
      do i = 1, n
        f2 = 2.0e+00 * float ( i ) - f1
        d2 = d2 * t2 * f2 / ( f2 + 1.0e+00 )
        ans = ans - d2
      end do
c
c  common return after computation.
c
4      continue

       if ( ans .lt. 0.0e+00 ) then
         ans = 0.0e+00
       end if

       return
c
c  df is an even integer.
c
5      continue

       d2 = t1 * sqrt ( t2 )
       ans = 1.0e+00 - d2

       if ( df .eq. 2 ) go to 4

       f1 = 1.0e+00
       go to 2
       end