SLATEC Routines --- SEPX4 ---
*DECK SEPX4
SUBROUTINE SEPX4 (IORDER, A, B, M, MBDCND, BDA, ALPHA, BDB, BETA,
+ C, D, N, NBDCND, BDC, BDD, COFX, GRHS, USOL, IDMN, W, PERTRB,
+ IERROR)
C***BEGIN PROLOGUE SEPX4
C***PURPOSE Solve for either the second or fourth order finite
C difference approximation to the solution of a separable
C elliptic partial differential equation on a rectangle.
C Any combination of periodic or mixed boundary conditions is
C allowed.
C***LIBRARY SLATEC (FISHPACK)
C***CATEGORY I2B1A2
C***TYPE SINGLE PRECISION (SEPX4-S)
C***KEYWORDS ELLIPTIC, FISHPACK, HELMHOLTZ, PDE, SEPARABLE
C***AUTHOR Adams, J., (NCAR)
C Swarztrauber, P. N., (NCAR)
C Sweet, R., (NCAR)
C***DESCRIPTION
C
C Purpose SEPX4 solves for either the second-order
C finite difference approximation or a
C fourth-order approximation to the
C solution of a separable elliptic equation
C AF(X)*UXX+BF(X)*UX+CF(X)*U+UYY = G(X,Y)
C
C on a rectangle (X greater than or equal to A
C and less than or equal to B; Y greater than
C or equal to C and less than or equal to D).
C Any combination of periodic or mixed boundary
C conditions is allowed.
C If boundary conditions in the X direction
C are periodic (see MBDCND=0 below) then the
C coefficients must satisfy
C AF(X)=C1,BF(X)=0,CF(X)=C2 for all X.
C Here C1,C2 are constants, C1.GT.0.
C
C The possible boundary conditions are
C in the X-direction:
C (0) Periodic, U(X+B-A,Y)=U(X,Y) for all Y,X
C (1) U(A,Y), U(B,Y) are specified for all Y
C (2) U(A,Y), dU(B,Y)/dX+BETA*U(B,Y) are
C specified for all Y
C (3) dU(A,Y)/dX+ALPHA*U(A,Y),dU(B,Y)/dX+
C BETA*U(B,Y) are specified for all Y
C (4) dU(A,Y)/dX+ALPHA*U(A,Y),U(B,Y) are
C specified for all Y
C
C In the Y-direction:
C (0) Periodic, U(X,Y+D-C)=U(X,Y) for all X,Y
C (1) U(X,C),U(X,D) are specified for all X
C (2) U(X,C),dU(X,D)/dY are specified for all X
C (3) dU(X,C)/DY,dU(X,D)/dY are specified for
C all X
C (4) dU(X,C)/DY,U(X,D) are specified for all X
C
C Usage Call SEPX4(IORDER,A,B,M,MBDCND,BDA,ALPHA,BDB,
C BETA,C,D,N,NBDCND,BDC,BDD,COFX,
C GRHS,USOL,IDMN,W,PERTRB,IERROR)
C
C Arguments
C
C IORDER
C = 2 If a second-order approximation is sought
C = 4 If a fourth-order approximation is sought
C
C A,B
C The range of the X-independent variable;
C i.e., X is greater than or equal to A and
C less than or equal to B. A must be less than
C B.
C
C M
C The number of panels into which the interval
C [A,B] is subdivided. Hence, there will be
C M+1 grid points in the X-direction given by
C XI=A+(I-1)*DLX for I=1,2,...,M+1 where
C DLX=(B-A)/M is the panel width. M must be
C less than IDMN and greater than 5.
C
C MBDCND
C Indicates the type of boundary condition at
C X=A and X=B
C = 0 If the solution is periodic in X; i.e.,
C U(X+B-A,Y)=U(X,Y) for all Y,X
C = 1 If the solution is specified at X=A and
C X=B; i.e., U(A,Y) and U(B,Y) are
C specified for all Y
C = 2 If the solution is specified at X=A and
C the boundary condition is mixed at X=B;
C i.e., U(A,Y) and dU(B,Y)/dX+BETA*U(B,Y)
C are specified for all Y
C = 3 If the boundary conditions at X=A and X=B
C are mixed; i.e., dU(A,Y)/dX+ALPHA*U(A,Y)
C and dU(B,Y)/dX+BETA*U(B,Y) are specified
C for all Y
C = 4 If the boundary condition at X=A is mixed
C and the solution is specified at X=B;
C i.e., dU(A,Y)/dX+ALPHA*U(A,Y) and U(B,Y)
C are specified for all Y
C
C BDA
C A one-dimensional array of length N+1 that
C specifies the values of dU(A,Y)/dX+
C ALPHA*U(A,Y) at X=A, when MBDCND=3 or 4.
C BDA(J) = dU(A,YJ)/dX+ALPHA*U(A,YJ);
C J=1,2,...,N+1
C When MBDCND has any other value, BDA is a
C dummy parameter.
C
C On Input ALPHA
C The scalar multiplying the solution in case
C of a mixed boundary condition AT X=A (see
C argument BDA). If MBDCND = 3,4 then ALPHA is
C a dummy parameter.
C
C BDB
C A one-dimensional array of length N+1 that
C specifies the values of dU(B,Y)/dX+
C BETA*U(B,Y) at X=B. when MBDCND=2 or 3
C BDB(J) = dU(B,YJ)/dX+BETA*U(B,YJ);
C J=1,2,...,N+1
C When MBDCND has any other value, BDB is a
C dummy parameter.
C
C BETA
C The scalar multiplying the solution in case
C of a mixed boundary condition at X=B (see
C argument BDB). If MBDCND=2,3 then BETA is a
C dummy parameter.
C
C C,D
C The range of the Y-independent variable;
C i.e., Y is greater than or equal to C and
C less than or equal to D. C must be less than
C D.
C
C N
C The number of panels into which the interval
C [C,D] is subdivided. Hence, there will be
C N+1 grid points in the Y-direction given by
C YJ=C+(J-1)*DLY for J=1,2,...,N+1 where
C DLY=(D-C)/N is the panel width. In addition,
C N must be greater than 4.
C
C NBDCND
C Indicates the types of boundary conditions at
C Y=C and Y=D
C = 0 If the solution is periodic in Y; i.e.,
C U(X,Y+D-C)=U(X,Y) for all X,Y
C = 1 If the solution is specified at Y=C and
C Y = D, i.e., U(X,C) and U(X,D) are
C specified for all X
C = 2 If the solution is specified at Y=C and
C the boundary condition is mixed at Y=D;
C i.e., dU(X,C)/dY and U(X,D)
C are specified for all X
C = 3 If the boundary conditions are mixed at
C Y= C and Y=D i.e., dU(X,D)/DY
C and dU(X,D)/dY are specified
C for all X
C = 4 If the boundary condition is mixed at Y=C
C and the solution is specified at Y=D;
C i.e. dU(X,C)/dY+GAMA*U(X,C) and U(X,D)
C are specified for all X
C
C BDC
C A one-dimensional array of length M+1 that
C specifies the value dU(X,C)/DY
C at Y=C. When NBDCND=3 or 4
C BDC(I) = dU(XI,C)/DY
C I=1,2,...,M+1.
C When NBDCND has any other value, BDC is a
C dummy parameter.
C
C
C BDD
C A one-dimensional array of length M+1 that
C specifies the value of dU(X,D)/DY
C at Y=D. When NBDCND=2 or 3
C BDD(I)=dU(XI,D)/DY
C I=1,2,...,M+1.
C When NBDCND has any other value, BDD is a
C dummy parameter.
C
C
C COFX
C A user-supplied subprogram with
C parameters X, AFUN, BFUN, CFUN which
C returns the values of the X-dependent
C coefficients AF(X), BF(X), CF(X) in
C the elliptic equation at X.
C If boundary conditions in the X direction
C are periodic then the coefficients
C must satisfy AF(X)=C1,BF(X)=0,CF(X)=C2 for
C all X. Here C1.GT.0 and C2 are constants.
C
C Note that COFX must be declared external
C in the calling routine.
C
C GRHS
C A two-dimensional array that specifies the
C values of the right-hand side of the elliptic
C equation; i.e., GRHS(I,J)=G(XI,YI), for
C I=2,...,M; J=2,...,N. At the boundaries,
C GRHS is defined by
C
C MBDCND GRHS(1,J) GRHS(M+1,J)
C ------ --------- -----------
C 0 G(A,YJ) G(B,YJ)
C 1 * *
C 2 * G(B,YJ) J=1,2,...,N+1
C 3 G(A,YJ) G(B,YJ)
C 4 G(A,YJ) *
C
C NBDCND GRHS(I,1) GRHS(I,N+1)
C ------ --------- -----------
C 0 G(XI,C) G(XI,D)
C 1 * *
C 2 * G(XI,D) I=1,2,...,M+1
C 3 G(XI,C) G(XI,D)
C 4 G(XI,C) *
C
C where * means these quantities are not used.
C GRHS should be dimensioned IDMN by at least
C N+1 in the calling routine.
C
C USOL
C A two-dimensional array that specifies the
C values of the solution along the boundaries.
C At the boundaries, USOL is defined by
C
C MBDCND USOL(1,J) USOL(M+1,J)
C ------ --------- -----------
C 0 * *
C 1 U(A,YJ) U(B,YJ)
C 2 U(A,YJ) * J=1,2,...,N+1
C 3 * *
C 4 * U(B,YJ)
C
C NBDCND USOL(I,1) USOL(I,N+1)
C ------ --------- -----------
C 0 * *
C 1 U(XI,C) U(XI,D)
C 2 U(XI,C) * I=1,2,...,M+1
C 3 * *
C 4 * U(XI,D)
C
C where * means the quantities are not used in
C the solution.
C
C If IORDER=2, the user may equivalence GRHS
C and USOL to save space. Note that in this
C case the tables specifying the boundaries of
C the GRHS and USOL arrays determine the
C boundaries uniquely except at the corners.
C If the tables call for both G(X,Y) and
C U(X,Y) at a corner then the solution must be
C chosen. For example, if MBDCND=2 and
C NBDCND=4, then U(A,C), U(A,D), U(B,D) must be
C chosen at the corners in addition to G(B,C).
C
C If IORDER=4, then the two arrays, USOL and
C GRHS, must be distinct.
C
C USOL should be dimensioned IDMN by at least
C N+1 in the calling routine.
C
C IDMN
C The row (or first) dimension of the arrays
C GRHS and USOL as it appears in the program
C calling SEPX4. This parameter is used to
C specify the variable dimension of GRHS and
C USOL. IDMN must be at least 7 and greater
C than or equal to M+1.
C
C W
C A one-dimensional array that must be provided
C by the user for work space.
C 10*N+(16+INT(log2(N)))*(M+1)+23 will suffice
C as a length for W. The actual length of
C W in the calling routine must be set in W(1)
C (see IERROR=11).
C
C On Output USOL
C Contains the approximate solution to the
C elliptic equation. USOL(I,J) is the
C approximation to U(XI,YJ) for I=1,2...,M+1
C and J=1,2,...,N+1. The approximation has
C error O(DLX**2+DLY**2) if called with
C IORDER=2 and O(DLX**4+DLY**4) if called with
C IORDER=4.
C
C W
C W(1) contains the exact minimal length (in
C floating point) required for the work space
C (see IERROR=11).
C
C PERTRB
C If a combination of periodic or derivative
C boundary conditions (i.e., ALPHA=BETA=0 if
C MBDCND=3) is specified and if CF(X)=0 for all
C X, then a solution to the discretized matrix
C equation may not exist (reflecting the non-
C uniqueness of solutions to the PDE). PERTRB
C is a constant calculated and subtracted from
C the right hand side of the matrix equation
C insuring the existence of a solution.
C SEPX4 computes this solution which is a
C weighted minimal least squares solution to
C the original problem. If singularity is
C not detected PERTRB=0.0 is returned by
C SEPX4.
C
C IERROR
C An error flag that indicates invalid input
C parameters or failure to find a solution
C = 0 No error
C = 1 If A greater than B or C greater than D
C = 2 If MBDCND less than 0 or MBDCND greater
C than 4
C = 3 If NBDCND less than 0 or NBDCND greater
C than 4
C = 4 If attempt to find a solution fails.
C (the linear system generated is not
C diagonally dominant.)
C = 5 If IDMN is too small (see discussion of
C IDMN)
C = 6 If M is too small or too large (see
C discussion of M)
C = 7 If N is too small (see discussion of N)
C = 8 If IORDER is not 2 or 4
C = 10 If AFUN is less than or equal to zero
C for some interior mesh point XI
C = 11 If the work space length input in W(1)
C is less than the exact minimal work
C space length required output in W(1).
C = 12 If MBDCND=0 and AF(X)=CF(X)=constant
C or BF(X)=0 for all X is not true.
C
C *Long Description:
C
C Dimension of BDA(N+1), BDB(N+1), BDC(M+1), BDD(M+1),
C Arguments USOL(IDMN,N+1), GRHS(IDMN,N+1),
C W (see argument list)
C
C Latest Revision October 1980
C
C Special Conditions NONE
C
C Common Blocks SPL4
C
C I/O NONE
C
C Precision Single
C
C Required Library NONE
C Files
C
C Specialist John C. Adams, NCAR, Boulder, Colorado 80307
C
C Language FORTRAN
C
C
C Entry Points SEPX4,SPELI4,CHKPR4,CHKSN4,ORTHO4,MINSO4,TRIS4,
C DEFE4,DX4,DY4
C
C History SEPX4 was developed by modifying the ULIB
C routine SEPELI during October 1978.
C It should be used instead of SEPELI whenever
C possible. The increase in speed is at least
C a factor of three.
C
C Algorithm SEPX4 automatically discretizes the separable
C elliptic equation which is then solved by a
C generalized cyclic reduction algorithm in the
C subroutine POIS. The fourth order solution
C is obtained using the technique of
C deferred corrections referenced below.
C
C
C References Keller, H.B., 'Numerical Methods for Two-point
C Boundary-value Problems', Blaisdel (1968),
C Waltham, Mass.
C
C Swarztrauber, P., and R. Sweet (1975):
C 'Efficient FORTRAN Subprograms For The
C Solution of Elliptic Partial Differential
C Equations'. NCAR Technical Note
C NCAR-TN/IA-109, pp. 135-137.
C
C***REFERENCES H. B. Keller, Numerical Methods for Two-point
C Boundary-value Problems, Blaisdel, Waltham, Mass.,
C 1968.
C P. N. Swarztrauber and R. Sweet, Efficient Fortran
C subprograms for the solution of elliptic equations,
C NCAR TN/IA-109, July 1975, 138 pp.
C***ROUTINES CALLED CHKPR4, SPELI4
C***REVISION HISTORY (YYMMDD)
C 801001 DATE WRITTEN
C 890531 Changed all specific intrinsics to generic. (WRB)
C 890531 REVISION DATE from Version 3.2
C 891214 Prologue converted to Version 4.0 format. (BAB)
C 920122 Minor corrections and modifications to prologue. (WRB)
C 920501 Reformatted the REFERENCES section. (WRB)
C***END PROLOGUE SEPX4