SRC/DEPRECATED/dlahrd.f(3)

🌸 NAME 🌸

SRC/DEPRECATED/dlahrd.f — okay bestie, this is the LAPACK Library Functions Manual page for SRC/DEPRECATED/dlahrd.f(3), and we’re folding the manual name right in here so you always know where you are. 🫶

✨ SYNOPSIS ✨

🧚 Functions/Subroutines

• subroutine dlahrd (n, k, nb, a, lda, tau, t, ldt, y, ldy) — DLAHRD reduces the first nb columns of a general rectangular matrix A so that elements below the k-th subdiagonal are zero, and returns auxiliary matrices which are needed to apply the transformation to the unreduced part of A.

💖 Function/Subroutine Documentation 💖

🦋 subroutine dlahrd (integer n, integer k, integer nb, double precision, dimension( lda, * ) a, integer lda, double precision, dimension( nb ) tau, double precision, dimension( ldt, nb ) t, integer ldt, double precision, dimension( ldy, nb ) y, integer ldy) 🦋

subroutine dlahrd (integer n, integer k, integer nb, double precision,
    dimension( lda, * ) a, integer lda, double precision, dimension( nb )
    tau, double precision, dimension( ldt, nb ) t, integer ldt, double
    precision, dimension( ldy, nb ) y, integer ldy)

DLAHRD reduces the first nb columns of a general rectangular matrix A so that elements below the k-th subdiagonal are zero, and returns auxiliary matrices which are needed to apply the transformation to the unreduced part of A.

🎯 Purpose:

Deprecated alert, bestie: This routine is deprecated and has been replaced by routine DLAHR2. No cap. 💅

DLAHRD reduces the first NB columns of a real general n-by-(n-k+1) matrix A so that elements below the k-th subdiagonal are zero. The reduction is performed by an orthogonal similarity transformation Q**T * A * Q. The routine returns the matrices V and T which determine Q as a block reflector I - V*T*V**T, and also the matrix Y = A * V * T.

🔍 Parameters

• N — N is INTEGER. The order of the matrix A.
• K — K is INTEGER. The offset for the reduction. Elements below the k-th subdiagonal in the first NB columns are reduced to zero.
• NB — NB is INTEGER. The number of columns to be reduced.
• A — A is DOUBLE PRECISION array, dimension (LDA,N-K+1). On entry, the n-by-(n-k+1) general matrix A. On exit, the elements on and above the k-th subdiagonal in the first NB columns are overwritten with the corresponding elements of the reduced matrix; the elements below the k-th subdiagonal, with the array TAU, represent the matrix Q as a product of elementary reflectors. The other columns of A are unchanged. See Further Details.
• LDA — LDA is INTEGER. The leading dimension of the array A. LDA >= max(1,N).
• TAU — TAU is DOUBLE PRECISION array, dimension (NB). The scalar factors of the elementary reflectors. See Further Details.
• T — T is DOUBLE PRECISION array, dimension (LDT,NB). The upper triangular matrix T.
• LDT — LDT is INTEGER. The leading dimension of the array T. LDT >= NB.
• Y — Y is DOUBLE PRECISION array, dimension (LDY,NB). The n-by-nb matrix Y.
• LDY — LDY is INTEGER. The leading dimension of the array Y. LDY >= N.

👑 Author

• Univ. of Tennessee
• Univ. of California Berkeley
• Univ. of Colorado Denver
• NAG Ltd.

📚 Further Details:

The matrix Q is represented as a product of nb elementary reflectors

Q = H(1) H(2) . . . H(nb).

Each H(i) has the form

H(i) = I - tau * v * v**T

where tau is a real scalar, and v is a real vector with v(1:i+k-1) = 0, v(i+k) = 1; v(i+k+1:n) is stored on exit in A(i+k+1:n,i), and tau in TAU(i).

The elements of the vectors v together form the (n-k+1)-by-nb matrix V which is needed, with T and Y, to apply the transformation to the unreduced part of the matrix, using an update of the form:

A := (I - V*T*V**T) * (A - Y*V**T).

The contents of A on exit are illustrated by the following example with n = 7, k = 3 and nb = 2:

( a   h   a   a   a )
( a   h   a   a   a )
( a   h   a   a   a )
( h   h   a   a   a )
( v1  h   a   a   a )
( v1  v2  a   a   a )
( v1  v2  a   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).

Definition at line 166 of file dlahrd.f.

🤖 Author

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That’s the whole page, bestie — LAPACK Version 3.12.0, source SRC/DEPRECATED/dlahrd.f(3). 💖✨