# slarfb_gett.f - Man Page

## Synopsis

### Functions/Subroutines

subroutine **slarfb_gett** (IDENT, M, **N**, K, T, LDT, A, **LDA**, B, **LDB**, WORK, LDWORK)**SLARFB_GETT**

## Function/Subroutine Documentation

### subroutine slarfb_gett (character IDENT, integer M, integer N, integer K, real, dimension( ldt, * ) T, integer LDT, real, dimension( lda, * ) A, integer LDA, real, dimension( ldb, * ) B, integer LDB, real, dimension( ldwork, * ) WORK, integer LDWORK)

**SLARFB_GETT**

**Purpose:**

SLARFB_GETT applies a real Householder block reflector H from the left to a real (K+M)-by-N "triangular-pentagonal" matrix composed of two block matrices: an upper trapezoidal K-by-N matrix A stored in the array A, and a rectangular M-by-(N-K) matrix B, stored in the array B. The block reflector H is stored in a compact WY-representation, where the elementary reflectors are in the arrays A, B and T. See Further Details section.

**Parameters***IDENT*IDENT is CHARACTER*1 If IDENT = not 'I', or not 'i', then V1 is unit lower-triangular and stored in the left K-by-K block of the input matrix A, If IDENT = 'I' or 'i', then V1 is an identity matrix and not stored. See Further Details section.

*M*M is INTEGER The number of rows of the matrix B. M >= 0.

*N*N is INTEGER The number of columns of the matrices A and B. N >= 0.

*K*K is INTEGER The number or rows of the matrix A. K is also order of the matrix T, i.e. the number of elementary reflectors whose product defines the block reflector. 0 <= K <= N.

*T*T is REAL array, dimension (LDT,K) The upper-triangular K-by-K matrix T in the representation of the block reflector.

*LDT*LDT is INTEGER The leading dimension of the array T. LDT >= K.

*A*A is REAL array, dimension (LDA,N) On entry: a) In the K-by-N upper-trapezoidal part A: input matrix A. b) In the columns below the diagonal: columns of V1 (ones are not stored on the diagonal). On exit: A is overwritten by rectangular K-by-N product H*A. See Further Details section.

*LDA*LDB is INTEGER The leading dimension of the array A. LDA >= max(1,K).

*B*B is REAL array, dimension (LDB,N) On entry: a) In the M-by-(N-K) right block: input matrix B. b) In the M-by-N left block: columns of V2. On exit: B is overwritten by rectangular M-by-N product H*B. See Further Details section.

*LDB*LDB is INTEGER The leading dimension of the array B. LDB >= max(1,M).

*WORK*WORK is REAL array, dimension (LDWORK,max(K,N-K))

*LDWORK*LDWORK is INTEGER The leading dimension of the array WORK. LDWORK>=max(1,K).

**Author**Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

**Contributors:**

November 2020, Igor Kozachenko, Computer Science Division, University of California, Berkeley

**Further Details:**

(1) Description of the Algebraic Operation. The matrix A is a K-by-N matrix composed of two column block matrices, A1, which is K-by-K, and A2, which is K-by-(N-K): A = ( A1, A2 ). The matrix B is an M-by-N matrix composed of two column block matrices, B1, which is M-by-K, and B2, which is M-by-(N-K): B = ( B1, B2 ). Perform the operation: ( A_out ) := H * ( A_in ) = ( I - V * T * V**T ) * ( A_in ) = ( B_out ) ( B_in ) ( B_in ) = ( I - ( V1 ) * T * ( V1**T, V2**T ) ) * ( A_in ) ( V2 ) ( B_in ) On input: a) ( A_in ) consists of two block columns: ( B_in ) ( A_in ) = (( A1_in ) ( A2_in )) = (( A1_in ) ( A2_in )) ( B_in ) (( B1_in ) ( B2_in )) (( 0 ) ( B2_in )), where the column blocks are: ( A1_in ) is a K-by-K upper-triangular matrix stored in the upper triangular part of the array A(1:K,1:K). ( B1_in ) is an M-by-K rectangular ZERO matrix and not stored. ( A2_in ) is a K-by-(N-K) rectangular matrix stored in the array A(1:K,K+1:N). ( B2_in ) is an M-by-(N-K) rectangular matrix stored in the array B(1:M,K+1:N). b) V = ( V1 ) ( V2 ) where: 1) if IDENT == 'I',V1 is a K-by-K identity matrix, not stored; 2) if IDENT != 'I',V1 is a K-by-K unit lower-triangular matrix, stored in the lower-triangular part of the array A(1:K,1:K) (ones are not stored), and V2 is an M-by-K rectangular stored the array B(1:M,1:K), (because on input B1_in is a rectangular zero matrix that is not stored and the space is used to store V2). c) T is a K-by-K upper-triangular matrix stored in the array T(1:K,1:K). On output: a) ( A_out ) consists of two block columns: ( B_out ) ( A_out ) = (( A1_out ) ( A2_out )) ( B_out ) (( B1_out ) ( B2_out )), where the column blocks are: ( A1_out ) is a K-by-K square matrix, or a K-by-K upper-triangular matrix, if V1 is an identity matrix. AiOut is stored in the array A(1:K,1:K). ( B1_out ) is an M-by-K rectangular matrix stored in the array B(1:M,K:N). ( A2_out ) is a K-by-(N-K) rectangular matrix stored in the array A(1:K,K+1:N). ( B2_out ) is an M-by-(N-K) rectangular matrix stored in the array B(1:M,K+1:N). The operation above can be represented as the same operation on each block column: ( A1_out ) := H * ( A1_in ) = ( I - V * T * V**T ) * ( A1_in ) ( B1_out ) ( 0 ) ( 0 ) ( A2_out ) := H * ( A2_in ) = ( I - V * T * V**T ) * ( A2_in ) ( B2_out ) ( B2_in ) ( B2_in ) If IDENT != 'I': The computation for column block 1: A1_out: = A1_in - V1*T*(V1**T)*A1_in B1_out: = - V2*T*(V1**T)*A1_in The computation for column block 2, which exists if N > K: A2_out: = A2_in - V1*T*( (V1**T)*A2_in + (V2**T)*B2_in ) B2_out: = B2_in - V2*T*( (V1**T)*A2_in + (V2**T)*B2_in ) If IDENT == 'I': The operation for column block 1: A1_out: = A1_in - V1*T**A1_in B1_out: = - V2*T**A1_in The computation for column block 2, which exists if N > K: A2_out: = A2_in - T*( A2_in + (V2**T)*B2_in ) B2_out: = B2_in - V2*T*( A2_in + (V2**T)*B2_in ) (2) Description of the Algorithmic Computation. In the first step, we compute column block 2, i.e. A2 and B2. Here, we need to use the K-by-(N-K) rectangular workspace matrix W2 that is of the same size as the matrix A2. W2 is stored in the array WORK(1:K,1:(N-K)). In the second step, we compute column block 1, i.e. A1 and B1. Here, we need to use the K-by-K square workspace matrix W1 that is of the same size as the as the matrix A1. W1 is stored in the array WORK(1:K,1:K). NOTE: Hence, in this routine, we need the workspace array WORK only of size WORK(1:K,1:max(K,N-K)) so it can hold both W2 from the first step and W1 from the second step. Case (A), when V1 is unit lower-triangular, i.e. IDENT != 'I', more computations than in the Case (B). if( IDENT != 'I' ) then if ( N > K ) then (First Step - column block 2) col2_(1) W2: = A2 col2_(2) W2: = (V1**T) * W2 = (unit_lower_tr_of_(A1)**T) * W2 col2_(3) W2: = W2 + (V2**T) * B2 = W2 + (B1**T) * B2 col2_(4) W2: = T * W2 col2_(5) B2: = B2 - V2 * W2 = B2 - B1 * W2 col2_(6) W2: = V1 * W2 = unit_lower_tr_of_(A1) * W2 col2_(7) A2: = A2 - W2 else (Second Step - column block 1) col1_(1) W1: = A1 col1_(2) W1: = (V1**T) * W1 = (unit_lower_tr_of_(A1)**T) * W1 col1_(3) W1: = T * W1 col1_(4) B1: = - V2 * W1 = - B1 * W1 col1_(5) square W1: = V1 * W1 = unit_lower_tr_of_(A1) * W1 col1_(6) square A1: = A1 - W1 end if end if Case (B), when V1 is an identity matrix, i.e. IDENT == 'I', less computations than in the Case (A) if( IDENT == 'I' ) then if ( N > K ) then (First Step - column block 2) col2_(1) W2: = A2 col2_(3) W2: = W2 + (V2**T) * B2 = W2 + (B1**T) * B2 col2_(4) W2: = T * W2 col2_(5) B2: = B2 - V2 * W2 = B2 - B1 * W2 col2_(7) A2: = A2 - W2 else (Second Step - column block 1) col1_(1) W1: = A1 col1_(3) W1: = T * W1 col1_(4) B1: = - V2 * W1 = - B1 * W1 col1_(6) upper-triangular_of_(A1): = A1 - W1 end if end if Combine these cases (A) and (B) together, this is the resulting algorithm: if ( N > K ) then (First Step - column block 2) col2_(1) W2: = A2 if( IDENT != 'I' ) then col2_(2) W2: = (V1**T) * W2 = (unit_lower_tr_of_(A1)**T) * W2 end if col2_(3) W2: = W2 + (V2**T) * B2 = W2 + (B1**T) * B2] col2_(4) W2: = T * W2 col2_(5) B2: = B2 - V2 * W2 = B2 - B1 * W2 if( IDENT != 'I' ) then col2_(6) W2: = V1 * W2 = unit_lower_tr_of_(A1) * W2 end if col2_(7) A2: = A2 - W2 else (Second Step - column block 1) col1_(1) W1: = A1 if( IDENT != 'I' ) then col1_(2) W1: = (V1**T) * W1 = (unit_lower_tr_of_(A1)**T) * W1 end if col1_(3) W1: = T * W1 col1_(4) B1: = - V2 * W1 = - B1 * W1 if( IDENT != 'I' ) then col1_(5) square W1: = V1 * W1 = unit_lower_tr_of_(A1) * W1 col1_(6_a) below_diag_of_(A1): = - below_diag_of_(W1) end if col1_(6_b) up_tr_of_(A1): = up_tr_of_(A1) - up_tr_of_(W1) end if

Definition at line 390 of file slarfb_gett.f.

## Author

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## Referenced By

The man page slarfb_gett(3) is an alias of slarfb_gett.f(3).

Thu Apr 1 2021 Version 3.9.1 LAPACK