SUBROUTINE USPCN(Y,A,N,G,H)
*     Calculation of B-spline coefficients A(N) from equidistant function
*     values in array Y(N), using the not-a-knot condition.
*     Arrays Y(*) and A(*) may be identical. 
*     G(N) and H(N) are auxiliary arrays used during computation. 
      REAL Y(N),A(N),G(N),H(N)
      DO I=2,N
       G(I)=Y(I)-Y(I-1)
      END DO
      H(1)=(5.0*G(2)+G(3))/2.0
      H(2)=-H(1)+3.0*(G(3)+G(2))
      G(2)=2.0
      DO I=3,N-1
       H(I)=-H(I-1)/G(I-1)+3.0*(G(I+1)+G(I))
       G(I)=4.0-1.0/G(I-1)
      END DO
      H(N)=(5.0*G(N)+Y(N-1)-Y(N-2))/2.0
      G(N)=1.0-2.0/G(N-1)
*     backward ...
      H(N)=(-2.0*H(N-1)/G(N-1)+H(N))/G(N)
      DO I=N-1,2,-1
       H(I)=(H(I)-H(I+1))/G(I)
      END DO
      H(1)=(H(1)-2.0*H(2))
*     ... and forward solution
      ALN=Y(N)+Y(N)-Y(N-1)-(2.0*H(N)+H(N-1))/3.0
      DO I=1,N-1
       A(I)=Y(I)+Y(I)-Y(I+1)+(2.0*H(I)+H(I+1))/3.0
      END DO
      A(N)=ALN
      END