?larft

Developer Reference for Intel® oneAPI Math Kernel Library for Fortran

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ID 766686

Date 10/15/2026

Version

Public

?larft

Forms the triangular factor T of a block reflector H = I - V * T * V ** H .

Syntax

call slarft(direct, storev, n, k, v, ldv, tau, t, ldt)
call dlarft(direct, storev, n, k, v, ldv, tau, t, ldt)
call clarft(direct, storev, n, k, v, ldv, tau, t, ldt)
call zlarft(direct, storev, n, k, v, ldv, tau, t, ldt)

Include Files

mkl.fi

Description

The routine ?larft forms the triangular factor T of a real/complex block reflector H of order n , which is defined as a product of k elementary reflectors.

If direct = 'F' , H = H(1)*H(2)* . . .*H(k) and T is upper triangular;

If direct = 'B' , H = H(k)*. . .*H(2)*H(1) and T is lower triangular.

If storev = 'C' , the vector which defines the elementary reflector H(i) is stored in the i -th column of the array v , and H = I - V*T*V^{T} (for real flavors) or H = I - V*T*V^{H} (for complex flavors) .

If storev = 'R' , the vector which defines the elementary reflector H(i) is stored in the i -th row of the array v , and H = I - V^{T}*T*V (for real flavors) or H = I - V^{H}*T*V (for complex flavors).

Input Parameters

The data types are given for the Fortran interface.

direct

CHARACTER*1 .

Specifies the order in which the elementary reflectors are multiplied to form the block reflector: = ‘F’ : H = H(1)*H(2)*. . . *H(k) (forward) = ‘B’ : H = H(k)*. . .*H(2)*H(1) (backward)

storev

CHARACTER*1 .

Specifies how the vectors which define the elementary reflectors are stored (see also Application Notes below) : = ‘C’ : column-wise = ‘R’ : row-wise.

n

INTEGER . The order of the block reflector H . n≥ 0 .

k

INTEGER . The order of the triangular factor T (equal to the number of elementary reflectors). k≥ 1 .

v: REAL for slarft DOUBLE PRECISION for dlarft COMPLEX for clarft DOUBLE COMPLEX for zlarft Array, DIMENSION ( ldv , k ) if storev = 'C' or ( ldv , n ) if storev = 'R' . The matrix V .
v: The size limitations depend on values of parameters storev and side as described in the following table: The matrix v . See Application Notes below.

ldv

INTEGER . The leading dimension of the array v .

If storev = 'C' , ldv≥ max(1,n)

if storev = 'R' , ldv≥k .

tau: REAL for slarft DOUBLE PRECISION for dlarft COMPLEX for clarft DOUBLE COMPLEX for zlarft Array, size ( k ). tau(i)[i-1] must contain the scalar factor of the elementary reflector H(i).

ldt

INTEGER . The leading dimension of the output array t . ldt≥k .

Output Parameters

t

REAL for slarft DOUBLE PRECISION for dlarft COMPLEX for clarft DOUBLE COMPLEX for zlarft

Array, size ldt by k . The k -by- k triangular factor T of the block reflector. If direct = 'F' , T is upper triangular; if direct = 'B' , T is lower triangular. The rest of the array is not used.

v

The matrix V .

Application Notes

The shape of the matrix \(V\) and the storage of the vectors which define the \(H(i)\) is best illustrated by the following example with n = 5 and k = 3 . The elements equal to 1 are not stored; the corresponding array elements are modified but restored on exit. The rest of the array is not used.

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Developer Reference for Intel® oneAPI Math Kernel Library for Fortran

?larft

Syntax

Include Files

Description

Input Parameters

Output Parameters

Application Notes