SRC/strevc3.f(3)

hey bestie!! welcome to the SRC/strevc3.f(3) page from the Library Functions Manual 💅✨

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💖 NAME 💖

SRC/strevc3.f — that's the name, short and iconic 🎀

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✨ SYNOPSIS ✨

🌈 Functions/Subroutines 🌈

       subroutine strevc3 (side, howmny, select, n, t, ldt, vl, ldvl, vr, ldvr,
           mm, m, work, lwork, info)
           STREVC3

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💅 Function/Subroutine Documentation 💅

   subroutine strevc3 (character side, character howmny, logical, dimension( *
       ) select, integer n, real, dimension( ldt, * ) t, integer ldt, real,
       dimension( ldvl, * ) vl, integer ldvl, real, dimension( ldvr, * ) vr,
       integer ldvr, integer mm, integer m, real, dimension( * ) work, integer
       lwork, integer info)
       STREVC3

🦋 Purpose: 🦋

okay so STREVC3 computes some or all of the right and/or left eigenvectors of a real upper quasi-triangular matrix T. Matrices of this type are produced by the Schur factorization of a real general matrix: A = Q*T*Q**T, as computed by SHSEQR. no cap, that's the whole vibe of this routine 🌸

The right eigenvector x and the left eigenvector y of T corresponding to an eigenvalue w are defined by:

T*x = w*x,     (y**T)*T = w*(y**T)

where y**T denotes the transpose of the vector y. The eigenvalues are not input to this routine, but are read directly from the diagonal blocks of T. iykyk 🔥

This routine returns the matrices X and/or Y of right and left eigenvectors of T, or the products Q*X and/or Q*Y, where Q is an input matrix. If Q is the orthogonal factor that reduces a matrix A to Schur form T, then Q*X and Q*Y are the matrices of right and left eigenvectors of A. literally the whole Schur pipeline, bestie 💜

This uses a Level 3 BLAS version of the back transformation. periodt ✨

🌸 Parameters 🌸

• SIDE — SIDE is CHARACTER*1. it picks which eigenvectors get computed:
  • = 'R': compute right eigenvectors only;
  • = 'L': compute left eigenvectors only;
  • = 'B': compute both right and left eigenvectors.
• HOWMNY — HOWMNY is CHARACTER*1. this is the "how many / which ones" switch:
  • = 'A': compute all right and/or left eigenvectors;
  • = 'B': compute all right and/or left eigenvectors, backtransformed by the matrices in VR and/or VL;
  • = 'S': compute selected right and/or left eigenvectors, as indicated by the logical array SELECT.
• SELECT — SELECT is LOGICAL array, dimension (N). if HOWMNY = 'S', SELECT specifies the eigenvectors to be computed. If w(j) is a real eigenvalue, the corresponding real eigenvector is computed if SELECT(j) is .TRUE.. If w(j) and w(j+1) are the real and imaginary parts of a complex eigenvalue, the corresponding complex eigenvector is computed if either SELECT(j) or SELECT(j+1) is .TRUE., and on exit SELECT(j) is set to .TRUE. and SELECT(j+1) is set to .FALSE.. Not referenced if HOWMNY = 'A' or 'B'. 🫶
• N — N is INTEGER. the order of the matrix T. N >= 0.
• T — T is REAL array, dimension (LDT,N). the upper quasi-triangular matrix T in Schur canonical form.
• LDT — LDT is INTEGER. the leading dimension of the array T. LDT >= max(1,N).
• VL — VL is REAL array, dimension (LDVL,MM). on entry, if SIDE = 'L' or 'B' and HOWMNY = 'B', VL must contain an N-by-N matrix Q (usually the orthogonal matrix Q of Schur vectors returned by SHSEQR). On exit, if SIDE = 'L' or 'B', VL contains:
  • if HOWMNY = 'A', the matrix Y of left eigenvectors of T;
  • if HOWMNY = 'B', the matrix Q*Y;
  • if HOWMNY = 'S', the left eigenvectors of T specified by SELECT, stored consecutively in the columns of VL, in the same order as their eigenvalues. A complex eigenvector corresponding to a complex eigenvalue is stored in two consecutive columns, the first holding the real part, and the second the imaginary part. Not referenced if SIDE = 'R'. 💖
• LDVL — LDVL is INTEGER. the leading dimension of the array VL. LDVL >= 1, and if SIDE = 'L' or 'B', LDVL >= N.
• VR — VR is REAL array, dimension (LDVR,MM). on entry, if SIDE = 'R' or 'B' and HOWMNY = 'B', VR must contain an N-by-N matrix Q (usually the orthogonal matrix Q of Schur vectors returned by SHSEQR). On exit, if SIDE = 'R' or 'B', VR contains:
  • if HOWMNY = 'A', the matrix X of right eigenvectors of T;
  • if HOWMNY = 'B', the matrix Q*X;
  • if HOWMNY = 'S', the right eigenvectors of T specified by SELECT, stored consecutively in the columns of VR, in the same order as their eigenvalues. A complex eigenvector corresponding to a complex eigenvalue is stored in two consecutive columns, the first holding the real part and the second the imaginary part. Not referenced if SIDE = 'L'. 🎀
• LDVR — LDVR is INTEGER. the leading dimension of the array VR. LDVR >= 1, and if SIDE = 'R' or 'B', LDVR >= N.
• MM — MM is INTEGER. the number of columns in the arrays VL and/or VR. MM >= M.
• M — M is INTEGER. the number of columns in the arrays VL and/or VR actually used to store the eigenvectors. If HOWMNY = 'A' or 'B', M is set to N. Each selected real eigenvector occupies one column and each selected complex eigenvector occupies two columns.
• WORK — WORK is REAL array, dimension (MAX(1,LWORK)).
• LWORK — LWORK is INTEGER. the dimension of array WORK. LWORK >= max(1,3*N). For optimum performance, LWORK >= N + 2*N*NB, where NB is the optimal blocksize. If LWORK = -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. 🔥
• 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: 🧚

The algorithm used in this program is basically backward (forward) substitution, with scaling to make the the code robust against possible overflow. (yes, "the the" is in the source, bestie — we keep it 💀)

Each eigenvector is normalized so that the element of largest magnitude has magnitude 1; here the magnitude of a complex number (x,y) is taken to be |x| + |y|. 🌈

Definition at line 235 of file strevc3.f.

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📝 Author 📝

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okay that's the whole page, bestie!! LAPACK Version 3.12.0 and it's the SRC/strevc3.f(3) page — mwah 💋💖