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NAME DLAHRD - reduce the first NB columns of a real general n- by-(n-k+1) matrix A so that elements below the k-th subdiag- onal are zero SYNOPSIS SUBROUTINE DLAHRD( N, K, NB, A, LDA, TAU, T, LDT, Y, LDY ) INTEGER K, LDA, LDT, LDY, N, NB DOUBLE PRECISION A( LDA, * ), T( LDT, NB ), TAU( NB ), Y( LDY, NB ) PURPOSE 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 simi- larity transformation Q' * A * Q. The routine returns the matrices V and T which determine Q as a block reflector I - V*T*V', and also the matrix Y = A * V * T. This is an auxiliary routine called by DGEHRD. ARGUMENTS N (input) INTEGER The order of the matrix A. K (input) INTEGER The offset for the reduction. Elements below the k- th subdiagonal in the first NB columns are reduced to zero. NB (input) INTEGER The number of columns to be reduced. K+1) A (input/output) DOUBLE PRECISION array, dimension (LDA,N- 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 elemen- tary reflectors. The other columns of A are unchanged. See Further Details. LDA (input) INTEGER The leading dimension of the array A. LDA >= max(1,N). TAU (output) DOUBLE PRECISION array, dimension (NB) The scalar factors of the elementary reflectors. See Further Details. T (output) DOUBLE PRECISION array, dimension (NB,NB) The upper triangular matrix T. LDT (input) INTEGER The leading dimension of the array T. LDT >= NB. Y (output) DOUBLE PRECISION array, dimension (LDY,NB) The n-by-nb matrix Y. LDY (input) INTEGER The leading dimension of the array Y. LDY >= N. 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' 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') * (A - Y*V'). 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).