SRC/zhetrd_2stage.f(3)

📦
version 3.12.1 · 7 versions
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💅 SRC/zhetrd_2stage.f — a whole entire LAPACK moment

okay bestie, so this is a page from the LAPACK library manual (the Library Functions Manual one, yes ma'am), and today's main character is a Fortran file called SRC/zhetrd_2stage.f. buckle up, because she does a LOT. 🎀

✨ SYNOPSIS ✨

Functions/Subroutines

  • subroutine zhetrd_2stage (vect, uplo, n, a, lda, d, e, tau, hous2, lhous2, work, lwork, info) — the icon herself, ZHETRD_2STAGE

🦋 Function/Subroutine Documentation 🦋

subroutine zhetrd_2stage (character vect, character uplo, integer n, complex*16, dimension( lda, * ) a, integer lda, double precision, dimension( * ) d, double precision, dimension( * ) e, complex*16, dimension( * ) tau, complex*16, dimension( * ) hous2, integer lhous2, complex*16, dimension( * ) work, integer lwork, integer info)

ZHETRD_2STAGE

🌸 Purpose 🌸

ok so the whole vibe: ZHETRD_2STAGE reduces a complex Hermitian matrix A to real symmetric tridiagonal form T by a unitary similarity transformation, and the transformation is literally:

Q1**H Q2**H* A * Q2 * Q1 = T.

no cap, that's the whole glow-up. 🔥

💖 Parameters 💖

  • VECT — VECT is CHARACTER*1
    • = 'N': no need for the Householder representation, in particular for the second stage (Band to tridiagonal), and thus LHOUS2 is of size max(1, 4*N);
    • = 'V': the Householder representation is needed to either generate Q1 Q2 or to apply Q1 Q2, then LHOUS2 is to be queried and computed. (NOT AVAILABLE IN THIS RELEASE).
🫶 yes bestie, the 'V' option is literally shouting "NOT AVAILABLE IN THIS RELEASE" — respect the all-caps, she means it.
  • UPLO — UPLO is CHARACTER*1
    • = 'U': upper triangle of A is stored;
    • = 'L': lower triangle of A is stored.
  • N — N is INTEGER
    • the order of the matrix A. N >= 0.
  • A — A is COMPLEX*16 array, dimension (LDA,N)
    • On entry, the Hermitian matrix A. If UPLO = 'U', the leading N-by-N upper triangular part of A contains the upper triangular part of the matrix A, and the strictly lower triangular part of A is not referenced. If UPLO = 'L', the leading N-by-N lower triangular part of A contains the lower triangular part of the matrix A, and the strictly upper triangular part of A is not referenced.
    • On exit, if UPLO = 'U', the band superdiagonal of A are overwritten by the corresponding elements of the internal band-diagonal matrix AB, and the elements above the KD superdiagonal, with the array TAU, represent the unitary matrix Q1 as a product of elementary reflectors; if UPLO = 'L', the diagonal and band subdiagonal of A are over-written by the corresponding elements of the internal band-diagonal matrix AB, and the elements below the KD subdiagonal, with the array TAU, represent the unitary matrix Q1 as a product of elementary reflectors. See Further Details.
🙈 she gets completely rewritten on exit. the matrix you knew is gone, that's just how it is.
  • LDA — LDA is INTEGER
    • the leading dimension of the array A. LDA >= max(1,N).
  • D — D is DOUBLE PRECISION array, dimension (N)
    • the diagonal elements of the tridiagonal matrix T. 🍓
  • E — E is DOUBLE PRECISION array, dimension (N-1)
    • the off-diagonal elements of the tridiagonal matrix T.
  • TAU — TAU is COMPLEX*16 array, dimension (N-KD)
    • the scalar factors of the elementary reflectors of the first stage (see Further Details).
  • HOUS2 — HOUS2 is COMPLEX*16 array, dimension (MAX(1,LHOUS2))
    • stores the Householder representation of the stage2 band to tridiagonal. 👑
  • LHOUS2 — LHOUS2 is INTEGER
    • the dimension of the array HOUS2. LHOUS2 >= 1.
    • If LWORK = -1, or LHOUS2 = -1, then a query is assumed; the routine only calculates the optimal size of the HOUS2 array, returns this value as the first entry of the HOUS2 array, and no error message related to LHOUS2 is issued by XERBLA.
    • If VECT='N', LHOUS2 = max(1, 4*n); if VECT='V', option not yet available.
  • WORK — WORK is COMPLEX*16 array, dimension (MAX(1,LWORK))
    • On exit, if INFO = 0, WORK(1) returns the optimal LWORK. ✨
  • LWORK — LWORK is INTEGER
    • the dimension of the array WORK. If N = 0, LWORK >= 1, else LWORK = MAX(1, dimension).
    • If LWORK = -1, or LHOUS2 = -1, then a workspace query is assumed; the routine only calculates the optimal size of the WORK array, returns this value as the first entry of the WORK array, and no error message related to LWORK is issued by XERBLA. LWORK = MAX(1, dimension) where
dimension   = max(stage1,stage2) + (KD+1)*N
            = N*KD + N*max(KD+1,FACTOPTNB)
              + max(2*KD*KD, KD*NTHREADS)
              + (KD+1)*N
  • where KD is the blocking size of the reduction, FACTOPTNB is the blocking used by the QR or LQ algorithm, usually FACTOPTNB=128 is a good choice, NTHREADS is the number of threads used when openMP compilation is enabled, otherwise =1.
😭 that formula is giving final boss energy but she's just telling you how much workspace to hand over. respect the math.
  • INFO — INFO is INTEGER
    • = 0: successful exit 🎉
    • < 0: if INFO = -i, the i-th argument had an illegal value 💀

🧚 Author 🧚

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

🌈 Further Details 🌈

  • Implemented by Azzam Haidar.
  • All details are available on technical report, SC11, SC13 papers.
  • Azzam Haidar, Hatem Ltaief, and Jack Dongarra. Parallel reduction to condensed forms for symmetric eigenvalue problems using aggregated fine-grained and memory-aware kernels. In Proceedings of 2011 International Conference for High Performance Computing, Networking, Storage and Analysis (SC '11), New York, NY, USA, Article 8 , 11 pages. http://doi.acm.org/10.1145/2063384.2063394
  • A. Haidar, J. Kurzak, P. Luszczek, 2013. An improved parallel singular value algorithm and its implementation for multicore hardware, In Proceedings of 2013 International Conference for High Performance Computing, Networking, Storage and Analysis (SC '13). Denver, Colorado, USA, 2013. Article 90, 12 pages. http://doi.acm.org/10.1145/2503210.2503292
  • A. Haidar, R. Solca, S. Tomov, T. Schulthess and J. Dongarra. A novel hybrid CPU-GPU generalized eigensolver for electronic structure calculations based on fine-grained memory aware tasks. International Journal of High Performance Computing Applications. Volume 28 Issue 2, Pages 196-209, May 2014. http://hpc.sagepub.com/content/28/2/196

📌 Definition 📌

Definition at line 227 of file zhetrd_2stage.f.

🫶 Author (the doxygen one) 🫶

Generated automatically by Doxygen for LAPACK from the source code.

okay that's the whole page bestie!! 🎀 this absolute queen lives in LAPACK, Version 3.12.0, and she was documented on SRC/zhetrd_2stage.f(3) — go forth and reduce some Hermitian matrices, ily, bye! 💜

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