`scipy.linalg._flapack:sstebz`

Signature

def sstebz(d, e, range, vl, vu, il, iu, tol, order)

Compute selected eigenvalues of a symmetric tridiagonal matrix by bisection (LAPACK sstebz).

d : ndarray: Diagonal elements of the symmetric tridiagonal matrix, length n.
e : ndarray: Off-diagonal elements, length n - 1.
range : int: Which eigenvalues to compute: 0 for all, 1 for those in (vl, vu], 2 for those with indices il through iu.
vl : float: Lower bound of the half-open interval; used only when range is 1.
vu : float: Upper bound of the half-open interval; used only when range is 1.
il : int: Index of the smallest eigenvalue to return, 1-based; used only when range is 2.
iu : int: Index of the largest eigenvalue to return, 1-based; used only when range is 2.
tol : float: Absolute tolerance for the eigenvalues. If not positive, machine precision times the matrix norm is used.
order : str: 'B' to order eigenvalues within each block, 'E' to order the whole set in ascending order.

m : int: Number of eigenvalues found; only w[:m] is meaningful.
w : ndarray: The computed eigenvalues, in the order selected by order.
iblock : ndarray: Block index of each eigenvalue: w[i] belongs to submatrix iblock[i], 1-based. A nonpositive entry marks an eigenvalue that failed to converge.
isplit : ndarray: Splitting points: submatrix i spans rows isplit[i-1] through isplit[i] - 1, 1-based.
info : int: 0 on success; if negative, the -info-th argument had an illegal value; if positive, some eigenvalues failed to converge or were not counted reliably.