SUBROUTINE CHPTRD( UPLO, N, AP, D, E, TAU, INFO ) * * -- LAPACK routine (version 3.2) -- * -- LAPACK is a software package provided by Univ. of Tennessee, -- * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..-- * November 2006 * * .. Scalar Arguments .. CHARACTER UPLO INTEGER INFO, N * .. * .. Array Arguments .. REAL D( * ), E( * ) COMPLEX AP( * ), TAU( * ) * .. * * Purpose * ======= * * CHPTRD reduces a complex Hermitian matrix A stored in packed form to * real symmetric tridiagonal form T by a unitary similarity * transformation: Q**H * 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. * * AP (input/output) COMPLEX array, dimension (N*(N+1)/2) * On entry, the upper or lower triangle of the Hermitian matrix * A, packed columnwise in a linear array. The j-th column of A * is stored in the array AP as follows: * if UPLO = 'U', AP(i + (j-1)*j/2) = A(i,j) for 1<=i<=j; * if UPLO = 'L', AP(i + (j-1)*(2*n-j)/2) = A(i,j) for j<=i<=n. * 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 unitary * 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 unitary matrix Q as a product * of elementary reflectors. See Further Details. * * 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) COMPLEX array, dimension (N-1) * The scalar factors of the elementary reflectors (see Further * Details). * * 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 complex scalar, and v is a complex vector with * v(i+1:n) = 0 and v(i) = 1; v(1:i-1) is stored on exit in AP, * overwriting A(1:i-1,i+1), and tau is stored 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 complex scalar, and v is a complex vector with * v(1:i) = 0 and v(i+1) = 1; v(i+2:n) is stored on exit in AP, * overwriting A(i+2:n,i), and tau is stored in TAU(i). * * ===================================================================== * * .. Parameters .. COMPLEX ONE, ZERO, HALF PARAMETER ( ONE = ( 1.0E+0, 0.0E+0 ), \$ ZERO = ( 0.0E+0, 0.0E+0 ), \$ HALF = ( 0.5E+0, 0.0E+0 ) ) * .. * .. Local Scalars .. LOGICAL UPPER INTEGER I, I1, I1I1, II COMPLEX ALPHA, TAUI * .. * .. External Subroutines .. EXTERNAL CAXPY, CHPMV, CHPR2, CLARFG, XERBLA * .. * .. External Functions .. LOGICAL LSAME COMPLEX CDOTC EXTERNAL LSAME, CDOTC * .. * .. Intrinsic Functions .. INTRINSIC REAL * .. * .. Executable Statements .. * * Test the input parameters * INFO = 0 UPPER = LSAME( UPLO, 'U' ) IF( .NOT.UPPER .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN INFO = -1 ELSE IF( N.LT.0 ) THEN INFO = -2 END IF IF( INFO.NE.0 ) THEN CALL XERBLA( 'CHPTRD', -INFO ) RETURN END IF * * Quick return if possible * IF( N.LE.0 ) \$ RETURN * IF( UPPER ) THEN * * Reduce the upper triangle of A. * I1 is the index in AP of A(1,I+1). * I1 = N*( N-1 ) / 2 + 1 AP( I1+N-1 ) = REAL( AP( I1+N-1 ) ) DO 10 I = N - 1, 1, -1 * * Generate elementary reflector H(i) = I - tau * v * v' * to annihilate A(1:i-1,i+1) * ALPHA = AP( I1+I-1 ) CALL CLARFG( I, ALPHA, AP( I1 ), 1, TAUI ) E( I ) = ALPHA * IF( TAUI.NE.ZERO ) THEN * * Apply H(i) from both sides to A(1:i,1:i) * AP( I1+I-1 ) = ONE * * Compute y := tau * A * v storing y in TAU(1:i) * CALL CHPMV( UPLO, I, TAUI, AP, AP( I1 ), 1, ZERO, TAU, \$ 1 ) * * Compute w := y - 1/2 * tau * (y'*v) * v * ALPHA = -HALF*TAUI*CDOTC( I, TAU, 1, AP( I1 ), 1 ) CALL CAXPY( I, ALPHA, AP( I1 ), 1, TAU, 1 ) * * Apply the transformation as a rank-2 update: * A := A - v * w' - w * v' * CALL CHPR2( UPLO, I, -ONE, AP( I1 ), 1, TAU, 1, AP ) * END IF AP( I1+I-1 ) = E( I ) D( I+1 ) = AP( I1+I ) TAU( I ) = TAUI I1 = I1 - I 10 CONTINUE D( 1 ) = AP( 1 ) ELSE * * Reduce the lower triangle of A. II is the index in AP of * A(i,i) and I1I1 is the index of A(i+1,i+1). * II = 1 AP( 1 ) = REAL( AP( 1 ) ) DO 20 I = 1, N - 1 I1I1 = II + N - I + 1 * * Generate elementary reflector H(i) = I - tau * v * v' * to annihilate A(i+2:n,i) * ALPHA = AP( II+1 ) CALL CLARFG( N-I, ALPHA, AP( II+2 ), 1, TAUI ) E( I ) = ALPHA * IF( TAUI.NE.ZERO ) THEN * * Apply H(i) from both sides to A(i+1:n,i+1:n) * AP( II+1 ) = ONE * * Compute y := tau * A * v storing y in TAU(i:n-1) * CALL CHPMV( UPLO, N-I, TAUI, AP( I1I1 ), AP( II+1 ), 1, \$ ZERO, TAU( I ), 1 ) * * Compute w := y - 1/2 * tau * (y'*v) * v * ALPHA = -HALF*TAUI*CDOTC( N-I, TAU( I ), 1, AP( II+1 ), \$ 1 ) CALL CAXPY( N-I, ALPHA, AP( II+1 ), 1, TAU( I ), 1 ) * * Apply the transformation as a rank-2 update: * A := A - v * w' - w * v' * CALL CHPR2( UPLO, N-I, -ONE, AP( II+1 ), 1, TAU( I ), 1, \$ AP( I1I1 ) ) * END IF AP( II+1 ) = E( I ) D( I ) = AP( II ) TAU( I ) = TAUI II = I1I1 20 CONTINUE D( N ) = AP( II ) END IF * RETURN * * End of CHPTRD * END