diff options
Diffstat (limited to 'SRC/ssytrd.f')
| -rw-r--r-- | SRC/ssytrd.f | 294 |
1 files changed, 294 insertions, 0 deletions
diff --git a/SRC/ssytrd.f b/SRC/ssytrd.f new file mode 100644 index 00000000..57a25239 --- /dev/null +++ b/SRC/ssytrd.f @@ -0,0 +1,294 @@ + SUBROUTINE SSYTRD( UPLO, N, A, LDA, D, E, TAU, WORK, LWORK, INFO ) +* +* -- LAPACK routine (version 3.1) -- +* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd.. +* November 2006 +* +* .. Scalar Arguments .. + CHARACTER UPLO + INTEGER INFO, LDA, LWORK, N +* .. +* .. Array Arguments .. + REAL A( LDA, * ), D( * ), E( * ), TAU( * ), + $ WORK( * ) +* .. +* +* Purpose +* ======= +* +* SSYTRD reduces a real symmetric matrix A to real symmetric +* tridiagonal form T by an orthogonal similarity transformation: +* Q**T * A * Q = T. +* +* Arguments +* ========= +* +* UPLO (input) CHARACTER*1 +* = 'U': Upper triangle of A is stored; +* = 'L': Lower triangle of A is stored. +* +* N (input) INTEGER +* The order of the matrix A. N >= 0. +* +* A (input/output) REAL array, dimension (LDA,N) +* On entry, the symmetric matrix A. If UPLO = 'U', the leading +* N-by-N upper triangular part of A contains the upper +* triangular part of the matrix A, and the strictly lower +* triangular part of A is not referenced. If UPLO = 'L', the +* leading N-by-N lower triangular part of A contains the lower +* triangular part of the matrix A, and the strictly upper +* triangular part of A is not referenced. +* On exit, if UPLO = 'U', the diagonal and first superdiagonal +* of A are overwritten by the corresponding elements of the +* tridiagonal matrix T, and the elements above the first +* superdiagonal, with the array TAU, represent the orthogonal +* matrix Q as a product of elementary reflectors; if UPLO +* = 'L', the diagonal and first subdiagonal of A are over- +* written by the corresponding elements of the tridiagonal +* matrix T, and the elements below the first subdiagonal, with +* the array TAU, represent the orthogonal matrix Q as a product +* of elementary reflectors. See Further Details. +* +* LDA (input) INTEGER +* The leading dimension of the array A. LDA >= max(1,N). +* +* D (output) REAL array, dimension (N) +* The diagonal elements of the tridiagonal matrix T: +* D(i) = A(i,i). +* +* E (output) REAL array, dimension (N-1) +* The off-diagonal elements of the tridiagonal matrix T: +* E(i) = A(i,i+1) if UPLO = 'U', E(i) = A(i+1,i) if UPLO = 'L'. +* +* TAU (output) REAL array, dimension (N-1) +* The scalar factors of the elementary reflectors (see Further +* Details). +* +* WORK (workspace/output) REAL array, dimension (MAX(1,LWORK)) +* On exit, if INFO = 0, WORK(1) returns the optimal LWORK. +* +* LWORK (input) INTEGER +* The dimension of the array WORK. LWORK >= 1. +* For optimum performance LWORK >= N*NB, where NB is the +* optimal blocksize. +* +* If LWORK = -1, then a workspace query is assumed; the routine +* only calculates the optimal size of the WORK array, returns +* this value as the first entry of the WORK array, and no error +* message related to LWORK is issued by XERBLA. +* +* INFO (output) INTEGER +* = 0: successful exit +* < 0: if INFO = -i, the i-th argument had an illegal value +* +* Further Details +* =============== +* +* If UPLO = 'U', the matrix Q is represented as a product of elementary +* reflectors +* +* Q = H(n-1) . . . H(2) H(1). +* +* Each H(i) has the form +* +* H(i) = I - tau * v * v' +* +* where tau is a real scalar, and v is a real vector with +* v(i+1:n) = 0 and v(i) = 1; v(1:i-1) is stored on exit in +* A(1:i-1,i+1), and tau in TAU(i). +* +* If UPLO = 'L', the matrix Q is represented as a product of elementary +* reflectors +* +* Q = H(1) H(2) . . . H(n-1). +* +* Each H(i) has the form +* +* H(i) = I - tau * v * v' +* +* where tau is a real scalar, and v is a real vector with +* v(1:i) = 0 and v(i+1) = 1; v(i+2:n) is stored on exit in A(i+2:n,i), +* and tau in TAU(i). +* +* The contents of A on exit are illustrated by the following examples +* with n = 5: +* +* if UPLO = 'U': if UPLO = 'L': +* +* ( d e v2 v3 v4 ) ( d ) +* ( d e v3 v4 ) ( e d ) +* ( d e v4 ) ( v1 e d ) +* ( d e ) ( v1 v2 e d ) +* ( d ) ( v1 v2 v3 e d ) +* +* where d and e denote diagonal and off-diagonal elements of T, and vi +* denotes an element of the vector defining H(i). +* +* ===================================================================== +* +* .. Parameters .. + REAL ONE + PARAMETER ( ONE = 1.0E+0 ) +* .. +* .. Local Scalars .. + LOGICAL LQUERY, UPPER + INTEGER I, IINFO, IWS, J, KK, LDWORK, LWKOPT, NB, + $ NBMIN, NX +* .. +* .. External Subroutines .. + EXTERNAL SLATRD, SSYR2K, SSYTD2, XERBLA +* .. +* .. Intrinsic Functions .. + INTRINSIC MAX +* .. +* .. External Functions .. + LOGICAL LSAME + INTEGER ILAENV + EXTERNAL LSAME, ILAENV +* .. +* .. Executable Statements .. +* +* Test the input parameters +* + INFO = 0 + UPPER = LSAME( UPLO, 'U' ) + LQUERY = ( LWORK.EQ.-1 ) + IF( .NOT.UPPER .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN + INFO = -1 + ELSE IF( N.LT.0 ) THEN + INFO = -2 + ELSE IF( LDA.LT.MAX( 1, N ) ) THEN + INFO = -4 + ELSE IF( LWORK.LT.1 .AND. .NOT.LQUERY ) THEN + INFO = -9 + END IF +* + IF( INFO.EQ.0 ) THEN +* +* Determine the block size. +* + NB = ILAENV( 1, 'SSYTRD', UPLO, N, -1, -1, -1 ) + LWKOPT = N*NB + WORK( 1 ) = LWKOPT + END IF +* + IF( INFO.NE.0 ) THEN + CALL XERBLA( 'SSYTRD', -INFO ) + RETURN + ELSE IF( LQUERY ) THEN + RETURN + END IF +* +* Quick return if possible +* + IF( N.EQ.0 ) THEN + WORK( 1 ) = 1 + RETURN + END IF +* + NX = N + IWS = 1 + IF( NB.GT.1 .AND. NB.LT.N ) THEN +* +* Determine when to cross over from blocked to unblocked code +* (last block is always handled by unblocked code). +* + NX = MAX( NB, ILAENV( 3, 'SSYTRD', UPLO, N, -1, -1, -1 ) ) + IF( NX.LT.N ) THEN +* +* Determine if workspace is large enough for blocked code. +* + LDWORK = N + IWS = LDWORK*NB + IF( LWORK.LT.IWS ) THEN +* +* Not enough workspace to use optimal NB: determine the +* minimum value of NB, and reduce NB or force use of +* unblocked code by setting NX = N. +* + NB = MAX( LWORK / LDWORK, 1 ) + NBMIN = ILAENV( 2, 'SSYTRD', UPLO, N, -1, -1, -1 ) + IF( NB.LT.NBMIN ) + $ NX = N + END IF + ELSE + NX = N + END IF + ELSE + NB = 1 + END IF +* + IF( UPPER ) THEN +* +* Reduce the upper triangle of A. +* Columns 1:kk are handled by the unblocked method. +* + KK = N - ( ( N-NX+NB-1 ) / NB )*NB + DO 20 I = N - NB + 1, KK + 1, -NB +* +* Reduce columns i:i+nb-1 to tridiagonal form and form the +* matrix W which is needed to update the unreduced part of +* the matrix +* + CALL SLATRD( UPLO, I+NB-1, NB, A, LDA, E, TAU, WORK, + $ LDWORK ) +* +* Update the unreduced submatrix A(1:i-1,1:i-1), using an +* update of the form: A := A - V*W' - W*V' +* + CALL SSYR2K( UPLO, 'No transpose', I-1, NB, -ONE, A( 1, I ), + $ LDA, WORK, LDWORK, ONE, A, LDA ) +* +* Copy superdiagonal elements back into A, and diagonal +* elements into D +* + DO 10 J = I, I + NB - 1 + A( J-1, J ) = E( J-1 ) + D( J ) = A( J, J ) + 10 CONTINUE + 20 CONTINUE +* +* Use unblocked code to reduce the last or only block +* + CALL SSYTD2( UPLO, KK, A, LDA, D, E, TAU, IINFO ) + ELSE +* +* Reduce the lower triangle of A +* + DO 40 I = 1, N - NX, NB +* +* Reduce columns i:i+nb-1 to tridiagonal form and form the +* matrix W which is needed to update the unreduced part of +* the matrix +* + CALL SLATRD( UPLO, N-I+1, NB, A( I, I ), LDA, E( I ), + $ TAU( I ), WORK, LDWORK ) +* +* Update the unreduced submatrix A(i+ib:n,i+ib:n), using +* an update of the form: A := A - V*W' - W*V' +* + CALL SSYR2K( UPLO, 'No transpose', N-I-NB+1, NB, -ONE, + $ A( I+NB, I ), LDA, WORK( NB+1 ), LDWORK, ONE, + $ A( I+NB, I+NB ), LDA ) +* +* Copy subdiagonal elements back into A, and diagonal +* elements into D +* + DO 30 J = I, I + NB - 1 + A( J+1, J ) = E( J ) + D( J ) = A( J, J ) + 30 CONTINUE + 40 CONTINUE +* +* Use unblocked code to reduce the last or only block +* + CALL SSYTD2( UPLO, N-I+1, A( I, I ), LDA, D( I ), E( I ), + $ TAU( I ), IINFO ) + END IF +* + WORK( 1 ) = LWKOPT + RETURN +* +* End of SSYTRD +* + END |