#include "blaswrap.h"
#include "f2c.h"

/* Subroutine */ int sgehd2_(integer *n, integer *ilo, integer *ihi, real *a, 
	integer *lda, real *tau, real *work, integer *info)
{
/*  -- LAPACK routine (version 3.0) --   
       Univ. of Tennessee, Univ. of California Berkeley, NAG Ltd.,   
       Courant Institute, Argonne National Lab, and Rice University   
       October 31, 1992   


    Purpose   
    =======   

    SGEHD2 reduces a real general matrix A to upper Hessenberg form H by   
    an orthogonal similarity transformation:  Q' * A * Q = H .   

    Arguments   
    =========   

    N       (input) INTEGER   
            The order of the matrix A.  N >= 0.   

    ILO     (input) INTEGER   
    IHI     (input) INTEGER   
            It is assumed that A is already upper triangular in rows   
            and columns 1:ILO-1 and IHI+1:N. ILO and IHI are normally   
            set by a previous call to SGEBAL; otherwise they should be   
            set to 1 and N respectively. See Further Details.   
            1 <= ILO <= IHI <= max(1,N).   

    A       (input/output) REAL array, dimension (LDA,N)   
            On entry, the n by n general matrix to be reduced.   
            On exit, the upper triangle and the first subdiagonal of A   
            are overwritten with the upper Hessenberg matrix H, 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).   

    TAU     (output) REAL array, dimension (N-1)   
            The scalar factors of the elementary reflectors (see Further   
            Details).   

    WORK    (workspace) REAL array, dimension (N)   

    INFO    (output) INTEGER   
            = 0:  successful exit.   
            < 0:  if INFO = -i, the i-th argument had an illegal value.   

    Further Details   
    ===============   

    The matrix Q is represented as a product of (ihi-ilo) elementary   
    reflectors   

       Q = H(ilo) H(ilo+1) . . . H(ihi-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, v(i+1) = 1 and v(ihi+1:n) = 0; v(i+2:ihi) is stored on   
    exit in A(i+2:ihi,i), and tau in TAU(i).   

    The contents of A are illustrated by the following example, with   
    n = 7, ilo = 2 and ihi = 6:   

    on entry,                        on exit,   

    ( a   a   a   a   a   a   a )    (  a   a   h   h   h   h   a )   
    (     a   a   a   a   a   a )    (      a   h   h   h   h   a )   
    (     a   a   a   a   a   a )    (      h   h   h   h   h   h )   
    (     a   a   a   a   a   a )    (      v2  h   h   h   h   h )   
    (     a   a   a   a   a   a )    (      v2  v3  h   h   h   h )   
    (     a   a   a   a   a   a )    (      v2  v3  v4  h   h   h )   
    (                         a )    (                          a )   

    where a denotes an element of the original matrix A, h denotes a   
    modified element of the upper Hessenberg matrix H, and vi denotes an   
    element of the vector defining H(i).   

    =====================================================================   


       Test the input parameters   

       Parameter adjustments */
    /* Table of constant values */
    static integer c__1 = 1;
    
    /* System generated locals */
    integer a_dim1, a_offset, i__1, i__2, i__3;
    /* Local variables */
    static integer i__;
    extern /* Subroutine */ int slarf_(char *, integer *, integer *, real *, 
	    integer *, real *, real *, integer *, real *), xerbla_(
	    char *, integer *), slarfg_(integer *, real *, real *, 
	    integer *, real *);
    static real aii;
#define a_ref(a_1,a_2) a[(a_2)*a_dim1 + a_1]


    a_dim1 = *lda;
    a_offset = 1 + a_dim1 * 1;
    a -= a_offset;
    --tau;
    --work;

    /* Function Body */
    *info = 0;
    if (*n < 0) {
	*info = -1;
    } else if (*ilo < 1 || *ilo > max(1,*n)) {
	*info = -2;
    } else if (*ihi < min(*ilo,*n) || *ihi > *n) {
	*info = -3;
    } else if (*lda < max(1,*n)) {
	*info = -5;
    }
    if (*info != 0) {
	i__1 = -(*info);
	xerbla_("SGEHD2", &i__1);
	return 0;
    }

    i__1 = *ihi - 1;
    for (i__ = *ilo; i__ <= i__1; ++i__) {

/*        Compute elementary reflector H(i) to annihilate A(i+2:ihi,i)   

   Computing MIN */
	i__2 = i__ + 2;
	i__3 = *ihi - i__;
	slarfg_(&i__3, &a_ref(i__ + 1, i__), &a_ref(min(i__2,*n), i__), &c__1,
		 &tau[i__]);
	aii = a_ref(i__ + 1, i__);
	a_ref(i__ + 1, i__) = 1.f;

/*        Apply H(i) to A(1:ihi,i+1:ihi) from the right */

	i__2 = *ihi - i__;
	slarf_("Right", ihi, &i__2, &a_ref(i__ + 1, i__), &c__1, &tau[i__], &
		a_ref(1, i__ + 1), lda, &work[1]);

/*        Apply H(i) to A(i+1:ihi,i+1:n) from the left */

	i__2 = *ihi - i__;
	i__3 = *n - i__;
	slarf_("Left", &i__2, &i__3, &a_ref(i__ + 1, i__), &c__1, &tau[i__], &
		a_ref(i__ + 1, i__ + 1), lda, &work[1]);

	a_ref(i__ + 1, i__) = aii;
/* L10: */
    }

    return 0;

/*     End of SGEHD2 */

} /* sgehd2_ */

#undef a_ref



.