SS2LT(3)

SYNOPSIS
SUBROUTINE SS2LT (N, NELT, IA, JA, A, ISYM, NEL, IEL, JEL, EL)

NAME
ss2lt - Lower Triangle Preconditioner SLAP Set Up. Routine to store the lower triangle of a matrix stored in the SLAP Column format.

LIBRARY
SLATEC (SLAP)

CATEGORY
D2E

TYPE
SINGLE PRECISION (SS2LT-S, DS2LT-D)

KEYWORDS
LINEAR SYSTEM, LOWER TRIANGLE, SLAP SPARSE

AUTHOR
Greenbaum, Anne, (Courant Institute) Seager, Mark K., (LLNL) Lawrence Livermore National Laboratory PO BOX 808, L-60 Livermore, CA 94550 (510) 423-3141 seager@llnl.gov

DESCRIPTION
*Usage: INTEGER N, NELT, IA(NELT), JA(NELT), ISYM INTEGER NEL, IEL(NEL), JEL(NEL) REAL A(NELT), EL(NEL) CALL SS2LT( N, NELT, IA, JA, A, ISYM, NEL, IEL, JEL, EL ) *Arguments: N :IN Integer Order of the Matrix. NELT :IN Integer. Number of non-zeros stored in A. IA :IN Integer IA(NELT). JA :IN Integer JA(NELT). A :IN Real A(NELT). These arrays should hold the matrix A in the SLAP Column format. See "Description", below. ISYM :IN Integer. Flag to indicate symmetric storage format. If ISYM=0, all non-zero entries of the matrix are stored. If ISYM=1, the matrix is symmetric, and only the lower triangle of the matrix is stored. NEL :OUT Integer. Number of non-zeros in the lower triangle of A. Also corresponds to the length of the IEL, JEL, EL arrays. IEL :OUT Integer IEL(NEL). JEL :OUT Integer JEL(NEL). EL :OUT Real EL(NEL). IEL, JEL, EL contain the lower triangle of the A matrix stored in SLAP Column format. See "Description", below, for more details bout the SLAP Column format. *Description =================== S L A P Column format ================== This routine requires that the matrix A be stored in the SLAP Column format. In this format the non-zeros are stored counting down columns (except for the diagonal entry, which must appear first in each "column") and are stored in the real array A. In other words, for each column in the matrix put the diagonal entry in A. Then put in the other non-zero elements going down the column (except the diagonal) in order. The IA array holds the row index for each non-zero. The JA array holds the offsets into the IA, A arrays for the beginning of each column. That is, IA(JA(ICOL)), A(JA(ICOL)) points to the beginning of the ICOL-th column in IA and A. IA(JA(ICOL+1)-1), A(JA(ICOL+1)-1) points to the end of the ICOL-th column. Note that we always have JA(N+1) = NELT+1, where N is the number of columns in the matrix and NELT is the number of non-zeros in the matrix. Here is an example of the SLAP Column storage format for a 5x5 Matrix (in the A and IA arrays '|' denotes the end of a column): 5x5 Matrix SLAP Column format for 5x5 matrix on left. 1 2 3 4 5 6 7 8 9 10 11 |11 12 0 0 15| A: 11 21 51 | 22 12 | 33 53 | 44 | 55 15 35 |21 22 0 0 0| IA: 1 2 5 | 2 1 | 3 5 | 4 | 5 1 3 | 0 0 33 0 35| JA: 1 4 6 8 9 12 | 0 0 0 44 0| |51 0 53 0 55|

REFERENCES
(NONE)

ROUTINES CALLED
(NONE)

REVISION HISTORY (YYMMDD)
871119 DATE WRITTEN
881213 Previous REVISION DATE
890915 Made changes requested at July 1989 CML Meeting. (MKS)
890922 Numerous changes to prologue to make closer to SLATEC standard. (FNF)
890929 Numerous changes to reduce SP/DP differences. (FNF)
910411 Prologue converted to Version 4.0 format. (BAB)
920511 Added complete declaration section. (WRB)
930701 Updated CATEGORY section. (FNF, WRB)