real function bvalue ( t, bcoef, n, k, x, jderiv )
c  from  * a practical guide to splines *  by c. de boor    
calls  interv
c
calculates value at  x  of  jderiv-th derivative of spline from b-repr.
c  the spline is taken to be continuous from the right, EXCEPT at the
c  rightmost knot, where it is taken to be continuous from the left.
c
c******  i n p u t ******
c  t, bcoef, n, k......forms the b-representation of the spline  f  to
c        be evaluated. specifically,
c  t.....knot sequence, of length  n+k, assumed nondecreasing.
c  bcoef.....b-coefficient sequence, of length  n .
c  n.....length of  bcoef  and dimension of spline(k,t),
c        a s s u m e d  positive .
c  k.....order of the spline .
c
c  w a r n i n g . . .   the restriction  k .le. kmax (=20)  is imposed
c        arbitrarily by the dimension statement for  aj, dl, dr  below,
c        but is  n o w h e r e  c h e c k e d  for.
c
c  x.....the point at which to evaluate .
c  jderiv.....integer giving the order of the derivative to be evaluated
c        a s s u m e d  to be zero or positive.
c
c******  o u t p u t  ******
c  bvalue.....the value of the (jderiv)-th derivative of  f  at  x .
c
c******  m e t h o d  ******
c     The nontrivial knot interval  (t(i),t(i+1))  containing  x  is lo-
c  cated with the aid of  interv . The  k  b-coeffs of  f  relevant for
c  this interval are then obtained from  bcoef (or taken to be zero if
c  not explicitly available) and are then differenced  jderiv  times to
c  obtain the b-coeffs of  (d**jderiv)f  relevant for that interval.
c  Precisely, with  j = jderiv, we have from x.(12) of the text that
c
c     (d**j)f  =  sum ( bcoef(.,j)*b(.,k-j,t) )
c
c  where
c                   / bcoef(.),                     ,  j .eq. 0
c                   /
c    bcoef(.,j)  =  / bcoef(.,j-1) - bcoef(.-1,j-1)
c                   / ----------------------------- ,  j .gt. 0
c                   /    (t(.+k-j) - t(.))/(k-j)
c
c     Then, we use repeatedly the fact that
c
c    sum ( a(.)*b(.,m,t)(x) )  =  sum ( a(.,x)*b(.,m-1,t)(x) )
c  with
c                 (x - t(.))*a(.) + (t(.+m-1) - x)*a(.-1)
c    a(.,x)  =    ---------------------------------------
c                 (x - t(.))      + (t(.+m-1) - x)
c
c  to write  (d**j)f(x)  eventually as a linear combination of b-splines
c  of order  1 , and the coefficient for  b(i,1,t)(x)  must then be the
c  desired number  (d**j)f(x). (see x.(17)-(19) of text).
c
      integer jderiv,k,n,   i,ilo,imk,j,jc,jcmin,jcmax,jj,kmax,kmj,km1
     *                     ,mflag,nmi,jdrvp1
      parameter (kmax = 20)
      real bcoef(n),t(n+k),x,   aj(kmax),dl(kmax),dr(kmax),fkmj
      bvalue = 0.
      if (jderiv .ge. k)                go to 99
c
c  *** Find  i   s.t.   1 .le. i .lt. n+k   and   t(i) .lt. t(i+1)   and
c      t(i) .le. x .lt. t(i+1) . If no such i can be found,  x  lies
c      outside the support of  the spline  f , hence  bvalue = 0.
c      (The asymmetry in this choice of  i  makes  f  rightcontinuous, except
c      at  t(n+k) where it is leftcontinuous.)
      call interv ( t, n+k, x, i, mflag )
      if (mflag .ne. 0)                 go to 99
c  *** if k = 1 (and jderiv = 0), bvalue = bcoef(i).
      km1 = k - 1
      if (km1 .gt. 0)                   go to 1
      bvalue = bcoef(i)
                                        go to 99
c
c  *** store the k b-spline coefficients relevant for the knot interval
c     (t(i),t(i+1)) in aj(1),...,aj(k) and compute dl(j) = x - t(i+1-j),
c     dr(j) = t(i+j) - x, j=1,...,k-1 . set any of the aj not obtainable
c     from input to zero. set any t.s not obtainable equal to t(1) or
c     to t(n+k) appropriately.
    1 jcmin = 1
      imk = i - k
      if (imk .ge. 0)                   go to 8
      jcmin = 1 - imk
      do 5 j=1,i
    5    dl(j) = x - t(i+1-j)
      do 6 j=i,km1
         aj(k-j) = 0.
    6    dl(j) = dl(i)
                                        go to 10
    8 do 9 j=1,km1
    9    dl(j) = x - t(i+1-j)
c
   10 jcmax = k
      nmi = n - i
      if (nmi .ge. 0)                   go to 18
      jcmax = k + nmi
      do 15 j=1,jcmax
   15    dr(j) = t(i+j) - x
      do 16 j=jcmax,km1
         aj(j+1) = 0.
   16    dr(j) = dr(jcmax)
                                        go to 20
   18 do 19 j=1,km1
   19    dr(j) = t(i+j) - x
c
   20 do 21 jc=jcmin,jcmax
   21    aj(jc) = bcoef(imk + jc)
c
c               *** difference the coefficients  jderiv  times.
      if (jderiv .eq. 0)                go to 30
      do 23 j=1,jderiv
         kmj = k-j
         fkmj = float(kmj)
         ilo = kmj
         do 23 jj=1,kmj
            aj(jj) = ((aj(jj+1) - aj(jj))/(dl(ilo) + dr(jj)))*fkmj
   23       ilo = ilo - 1
c
c  *** compute value at  x  in (t(i),t(i+1)) of jderiv-th derivative,
c     given its relevant b-spline coeffs in aj(1),...,aj(k-jderiv).
   30 if (jderiv .eq. km1)              go to 39
      jdrvp1 = jderiv + 1     
      do 33 j=jdrvp1,km1
         kmj = k-j
         ilo = kmj
         do 33 jj=1,kmj
            aj(jj) = (aj(jj+1)*dl(ilo) + aj(jj)*dr(jj))/(dl(ilo)+dr(jj))
   33       ilo = ilo - 1
   39 bvalue = aj(1)
c
   99                                   return
      end