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LAPACK 3.12.1
LAPACK: Linear Algebra PACKage
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LAPACK 3.12.1
LAPACK: Linear Algebra PACKage
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| subroutine slaebz | ( | integer | ijob, |
| integer | nitmax, | ||
| integer | n, | ||
| integer | mmax, | ||
| integer | minp, | ||
| integer | nbmin, | ||
| real | abstol, | ||
| real | reltol, | ||
| real | pivmin, | ||
| real, dimension( * ) | d, | ||
| real, dimension( * ) | e, | ||
| real, dimension( * ) | e2, | ||
| integer, dimension( * ) | nval, | ||
| real, dimension( mmax, * ) | ab, | ||
| real, dimension( * ) | c, | ||
| integer | mout, | ||
| integer, dimension( mmax, * ) | nab, | ||
| real, dimension( * ) | work, | ||
| integer, dimension( * ) | iwork, | ||
| integer | info ) |
SLAEBZ computes the number of eigenvalues of a real symmetric tridiagonal matrix which are less than or equal to a given value, and performs other tasks required by the routine sstebz.
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!> !> SLAEBZ contains the iteration loops which compute and use the !> function N(w), which is the count of eigenvalues of a symmetric !> tridiagonal matrix T less than or equal to its argument w. It !> performs a choice of two types of loops: !> !> IJOB=1, followed by !> IJOB=2: It takes as input a list of intervals and returns a list of !> sufficiently small intervals whose union contains the same !> eigenvalues as the union of the original intervals. !> The input intervals are (AB(j,1),AB(j,2)], j=1,...,MINP. !> The output interval (AB(j,1),AB(j,2)] will contain !> eigenvalues NAB(j,1)+1,...,NAB(j,2), where 1 <= j <= MOUT. !> !> IJOB=3: It performs a binary search in each input interval !> (AB(j,1),AB(j,2)] for a point w(j) such that !> N(w(j))=NVAL(j), and uses C(j) as the starting point of !> the search. If such a w(j) is found, then on output !> AB(j,1)=AB(j,2)=w. If no such w(j) is found, then on output !> (AB(j,1),AB(j,2)] will be a small interval containing the !> point where N(w) jumps through NVAL(j), unless that point !> lies outside the initial interval. !> !> Note that the intervals are in all cases half-open intervals, !> i.e., of the form (a,b] , which includes b but not a . !> !> To avoid underflow, the matrix should be scaled so that its largest !> element is no greater than overflow**(1/2) * underflow**(1/4) !> in absolute value. To assure the most accurate computation !> of small eigenvalues, the matrix should be scaled to be !> not much smaller than that, either. !> !> See W. Kahan , Report CS41, Computer Science Dept., Stanford !> University, July 21, 1966 !> !> Note: the arguments are, in general, *not* checked for unreasonable !> values. !>
| [in] | IJOB | !> IJOB is INTEGER !> Specifies what is to be done: !> = 1: Compute NAB for the initial intervals. !> = 2: Perform bisection iteration to find eigenvalues of T. !> = 3: Perform bisection iteration to invert N(w), i.e., !> to find a point which has a specified number of !> eigenvalues of T to its left. !> Other values will cause SLAEBZ to return with INFO=-1. !> |
| [in] | NITMAX | !> NITMAX is INTEGER !> The maximum number of of bisection to be !> performed, i.e., an interval of width W will not be made !> smaller than 2^(-NITMAX) * W. If not all intervals !> have converged after NITMAX iterations, then INFO is set !> to the number of non-converged intervals. !> |
| [in] | N | !> N is INTEGER !> The dimension n of the tridiagonal matrix T. It must be at !> least 1. !> |
| [in] | MMAX | !> MMAX is INTEGER !> The maximum number of intervals. If more than MMAX intervals !> are generated, then SLAEBZ will quit with INFO=MMAX+1. !> |
| [in] | MINP | !> MINP is INTEGER !> The initial number of intervals. It may not be greater than !> MMAX. !> |
| [in] | NBMIN | !> NBMIN is INTEGER !> The smallest number of intervals that should be processed !> using a vector loop. If zero, then only the scalar loop !> will be used. !> |
| [in] | ABSTOL | !> ABSTOL is REAL !> The minimum (absolute) width of an interval. When an !> interval is narrower than ABSTOL, or than RELTOL times the !> larger (in magnitude) endpoint, then it is considered to be !> sufficiently small, i.e., converged. This must be at least !> zero. !> |
| [in] | RELTOL | !> RELTOL is REAL !> The minimum relative width of an interval. When an interval !> is narrower than ABSTOL, or than RELTOL times the larger (in !> magnitude) endpoint, then it is considered to be !> sufficiently small, i.e., converged. Note: this should !> always be at least radix*machine epsilon. !> |
| [in] | PIVMIN | !> PIVMIN is REAL !> The minimum absolute value of a in the Sturm !> sequence loop. !> This must be at least max |e(j)**2|*safe_min and at !> least safe_min, where safe_min is at least !> the smallest number that can divide one without overflow. !> |
| [in] | D | !> D is REAL array, dimension (N) !> The diagonal elements of the tridiagonal matrix T. !> |
| [in] | E | !> E is REAL array, dimension (N) !> The offdiagonal elements of the tridiagonal matrix T in !> positions 1 through N-1. E(N) is arbitrary. !> |
| [in] | E2 | !> E2 is REAL array, dimension (N) !> The squares of the offdiagonal elements of the tridiagonal !> matrix T. E2(N) is ignored. !> |
| [in,out] | NVAL | !> NVAL is INTEGER array, dimension (MINP) !> If IJOB=1 or 2, not referenced. !> If IJOB=3, the desired values of N(w). The elements of NVAL !> will be reordered to correspond with the intervals in AB. !> Thus, NVAL(j) on output will not, in general be the same as !> NVAL(j) on input, but it will correspond with the interval !> (AB(j,1),AB(j,2)] on output. !> |
| [in,out] | AB | !> AB is REAL array, dimension (MMAX,2) !> The endpoints of the intervals. AB(j,1) is a(j), the left !> endpoint of the j-th interval, and AB(j,2) is b(j), the !> right endpoint of the j-th interval. The input intervals !> will, in general, be modified, split, and reordered by the !> calculation. !> |
| [in,out] | C | !> C is REAL array, dimension (MMAX) !> If IJOB=1, ignored. !> If IJOB=2, workspace. !> If IJOB=3, then on input C(j) should be initialized to the !> first search point in the binary search. !> |
| [out] | MOUT | !> MOUT is INTEGER !> If IJOB=1, the number of eigenvalues in the intervals. !> If IJOB=2 or 3, the number of intervals output. !> If IJOB=3, MOUT will equal MINP. !> |
| [in,out] | NAB | !> NAB is INTEGER array, dimension (MMAX,2) !> If IJOB=1, then on output NAB(i,j) will be set to N(AB(i,j)). !> If IJOB=2, then on input, NAB(i,j) should be set. It must !> satisfy the condition: !> N(AB(i,1)) <= NAB(i,1) <= NAB(i,2) <= N(AB(i,2)), !> which means that in interval i only eigenvalues !> NAB(i,1)+1,...,NAB(i,2) will be considered. Usually, !> NAB(i,j)=N(AB(i,j)), from a previous call to SLAEBZ with !> IJOB=1. !> On output, NAB(i,j) will contain !> max(na(k),min(nb(k),N(AB(i,j)))), where k is the index of !> the input interval that the output interval !> (AB(j,1),AB(j,2)] came from, and na(k) and nb(k) are the !> the input values of NAB(k,1) and NAB(k,2). !> If IJOB=3, then on output, NAB(i,j) contains N(AB(i,j)), !> unless N(w) > NVAL(i) for all search points w , in which !> case NAB(i,1) will not be modified, i.e., the output !> value will be the same as the input value (modulo !> reorderings -- see NVAL and AB), or unless N(w) < NVAL(i) !> for all search points w , in which case NAB(i,2) will !> not be modified. Normally, NAB should be set to some !> distinctive value(s) before SLAEBZ is called. !> |
| [out] | WORK | !> WORK is REAL array, dimension (MMAX) !> Workspace. !> |
| [out] | IWORK | !> IWORK is INTEGER array, dimension (MMAX) !> Workspace. !> |
| [out] | INFO | !> INFO is INTEGER !> = 0: All intervals converged. !> = 1--MMAX: The last INFO intervals did not converge. !> = MMAX+1: More than MMAX intervals were generated. !> |
!> !> This routine is intended to be called only by other LAPACK !> routines, thus the interface is less user-friendly. It is intended !> for two purposes: !> !> (a) finding eigenvalues. In this case, SLAEBZ should have one or !> more initial intervals set up in AB, and SLAEBZ should be called !> with IJOB=1. This sets up NAB, and also counts the eigenvalues. !> Intervals with no eigenvalues would usually be thrown out at !> this point. Also, if not all the eigenvalues in an interval i !> are desired, NAB(i,1) can be increased or NAB(i,2) decreased. !> For example, set NAB(i,1)=NAB(i,2)-1 to get the largest !> eigenvalue. SLAEBZ is then called with IJOB=2 and MMAX !> no smaller than the value of MOUT returned by the call with !> IJOB=1. After this (IJOB=2) call, eigenvalues NAB(i,1)+1 !> through NAB(i,2) are approximately AB(i,1) (or AB(i,2)) to the !> tolerance specified by ABSTOL and RELTOL. !> !> (b) finding an interval (a',b'] containing eigenvalues w(f),...,w(l). !> In this case, start with a Gershgorin interval (a,b). Set up !> AB to contain 2 search intervals, both initially (a,b). One !> NVAL element should contain f-1 and the other should contain l !> , while C should contain a and b, resp. NAB(i,1) should be -1 !> and NAB(i,2) should be N+1, to flag an error if the desired !> interval does not lie in (a,b). SLAEBZ is then called with !> IJOB=3. On exit, if w(f-1) < w(f), then one of the intervals -- !> j -- will have AB(j,1)=AB(j,2) and NAB(j,1)=NAB(j,2)=f-1, while !> if, to the specified tolerance, w(f-k)=...=w(f+r), k > 0 and r !> >= 0, then the interval will have N(AB(j,1))=NAB(j,1)=f-k and !> N(AB(j,2))=NAB(j,2)=f+r. The cases w(l) < w(l+1) and !> w(l-r)=...=w(l+k) are handled similarly. !>
Definition at line 314 of file slaebz.f.
1.11.0