*DECK GENBUN SUBROUTINE GENBUN (NPEROD, N, MPEROD, M, A, B, C, IDIMY, Y, + IERROR, W) C***BEGIN PROLOGUE GENBUN C***PURPOSE Solve by a cyclic reduction algorithm the linear system C of equations that results from a finite difference C approximation to certain 2-d elliptic PDE's on a centered C grid . C***LIBRARY SLATEC (FISHPACK) C***CATEGORY I2B4B C***TYPE SINGLE PRECISION (GENBUN-S, CMGNBN-C) C***KEYWORDS ELLIPTIC, FISHPACK, PDE, TRIDIAGONAL C***AUTHOR Adams, J., (NCAR) C Swarztrauber, P. N., (NCAR) C Sweet, R., (NCAR) C***DESCRIPTION C C Subroutine GENBUN solves the linear system of equations C C A(I)*X(I-1,J) + B(I)*X(I,J) + C(I)*X(I+1,J) C C + X(I,J-1) - 2.*X(I,J) + X(I,J+1) = Y(I,J) C C for I = 1,2,...,M and J = 1,2,...,N. C C The indices I+1 and I-1 are evaluated modulo M, i.e., C X(0,J) = X(M,J) and X(M+1,J) = X(1,J), and X(I,0) may be equal to C 0, X(I,2), or X(I,N) and X(I,N+1) may be equal to 0, X(I,N-1), or C X(I,1) depending on an input parameter. C C C * * * * * * * * Parameter Description * * * * * * * * * * C C * * * * * * On Input * * * * * * C C NPEROD C Indicates the values that X(I,0) and X(I,N+1) are assumed to C have. C C = 0 If X(I,0) = X(I,N) and X(I,N+1) = X(I,1). C = 1 If X(I,0) = X(I,N+1) = 0 . C = 2 If X(I,0) = 0 and X(I,N+1) = X(I,N-1). C = 3 If X(I,0) = X(I,2) and X(I,N+1) = X(I,N-1). C = 4 If X(I,0) = X(I,2) and X(I,N+1) = 0. C C N C The number of unknowns in the J-direction. N must be greater C than 2. C C MPEROD C = 0 if A(1) and C(M) are not zero. C = 1 if A(1) = C(M) = 0. C C M C The number of unknowns in the I-direction. M must be greater C than 2. C C A,B,C C One-dimensional arrays of length M that specify the C coefficients in the linear equations given above. If MPEROD = 0 C the array elements must not depend upon the index I, but must be C constant. Specifically, the subroutine checks the following C condition C C A(I) = C(1) C C(I) = C(1) C B(I) = B(1) C C for I=1,2,...,M. C C IDIMY C The row (or first) dimension of the two-dimensional array Y as C it appears in the program calling GENBUN. This parameter is C used to specify the variable dimension of Y. IDIMY must be at C least M. C C Y C A two-dimensional array that specifies the values of the right C side of the linear system of equations given above. Y must be C dimensioned at least M*N. C C W C A one-dimensional array that must be provided by the user for C work space. W may require up to 4*N + (10 + INT(log2(N)))*M C locations. The actual number of locations used is computed by C GENBUN and is returned in location W(1). C C C * * * * * * On Output * * * * * * C C Y C Contains the solution X. C C IERROR C An error flag that indicates invalid input parameters. Except C for number zero, a solution is not attempted. C C = 0 No error. C = 1 M .LE. 2 C = 2 N .LE. 2 C = 3 IDIMY .LT. M C = 4 NPEROD .LT. 0 or NPEROD .GT. 4 C = 5 MPEROD .LT. 0 or MPEROD .GT. 1 C = 6 A(I) .NE. C(1) or C(I) .NE. C(1) or B(I) .NE. B(1) for C some I=1,2,...,M. C = 7 A(1) .NE. 0 or C(M) .NE. 0 and MPEROD = 1 C C W C W(1) contains the required length of W. C C *Long Description: C C * * * * * * * Program Specifications * * * * * * * * * * * * C C Dimension of A(M),B(M),C(M),Y(IDIMY,N),W(see parameter list) C Arguments C C Latest June 1, 1976 C Revision C C Subprograms GENBUN,POISD2,POISN2,POISP2,COSGEN,MERGE,TRIX,TRI3, C Required PIMACH C C Special NONE C Conditions C C Common NONE C Blocks C C I/O NONE C C Precision Single C C Specialist Roland Sweet C C Language FORTRAN C C History Standardized April 1, 1973 C Revised August 20,1973 C Revised January 1, 1976 C C Algorithm The linear system is solved by a cyclic reduction C algorithm described in the reference. C C Space 4944(decimal) = 11520(octal) locations on the NCAR C Required Control Data 7600. C C Timing and The execution time T on the NCAR Control Data C Accuracy 7600 for subroutine GENBUN is roughly proportional C to M*N*log2(N), but also depends on the input C parameter NPEROD. Some typical values are listed C in the table below. More comprehensive timing C charts may be found in the reference. C To measure the accuracy of the algorithm a C uniform random number generator was used to create C a solution array X for the system given in the C 'PURPOSE' with C C A(I) = C(I) = -0.5*B(I) = 1, I=1,2,...,M C C and, when MPEROD = 1 C C A(1) = C(M) = 0 C A(M) = C(1) = 2. C C The solution X was substituted into the given sys- C tem and, using double precision, a right side Y was C computed. Using this array Y subroutine GENBUN was C called to produce an approximate solution Z. Then C the relative error, defined as C C E = MAX(ABS(Z(I,J)-X(I,J)))/MAX(ABS(X(I,J))) C C where the two maxima are taken over all I=1,2,...,M C and J=1,2,...,N, was computed. The value of E is C given in the table below for some typical values of C M and N. C C C M (=N) MPEROD NPEROD T(MSECS) E C ------ ------ ------ -------- ------ C C 31 0 0 36 6.E-14 C 31 1 1 21 4.E-13 C 31 1 3 41 3.E-13 C 32 0 0 29 9.E-14 C 32 1 1 32 3.E-13 C 32 1 3 48 1.E-13 C 33 0 0 36 9.E-14 C 33 1 1 30 4.E-13 C 33 1 3 34 1.E-13 C 63 0 0 150 1.E-13 C 63 1 1 91 1.E-12 C 63 1 3 173 2.E-13 C 64 0 0 122 1.E-13 C 64 1 1 128 1.E-12 C 64 1 3 199 6.E-13 C 65 0 0 143 2.E-13 C 65 1 1 120 1.E-12 C 65 1 3 138 4.E-13 C C Portability American National Standards Institute Fortran. C The machine dependent constant PI is defined in C function PIMACH. C C Required COS C Resident C Routines C C Reference Sweet, R., 'A Cyclic Reduction Algorithm For C Solving Block Tridiagonal Systems Of Arbitrary C Dimensions,' SIAM J. on Numer. Anal., C 14(Sept., 1977), PP. 706-720. C C * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * C C***REFERENCES R. Sweet, A cyclic reduction algorithm for solving C block tridiagonal systems of arbitrary dimensions, C SIAM Journal on Numerical Analysis 14, (September C 1977), pp. 706-720. C***ROUTINES CALLED POISD2, POISN2, POISP2 C***REVISION HISTORY (YYMMDD) C 801001 DATE WRITTEN C 861211 REVISION DATE from Version 3.2 C 891214 Prologue converted to Version 4.0 format. (BAB) C 920501 Reformatted the REFERENCES section. (WRB) C***END PROLOGUE GENBUN