init

2026-02-12 23:22:11 +08:00
parent 7b09eb3d89
commit 89660bba4e
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+"""
+Matrix square root for general matrices and for upper triangular matrices.
+
+This module exists to avoid cyclic imports.
+
+"""
+__all__ = []
+
+import numpy as np
+
+# Local imports
+from .lapack import ztrsyl, dtrsyl
+
+class SqrtmError(np.linalg.LinAlgError):
+    pass
+
+from ._matfuncs_sqrtm_triu import within_block_loop  # noqa: E402
+
+
+def _sqrtm_triu(T, blocksize=64):
+    """
+    Matrix square root of an upper triangular matrix.
+
+    This is a helper function for `sqrtm` and `logm`.
+
+    Parameters
+    ----------
+    T : (N, N) array_like upper triangular
+        Matrix whose square root to evaluate
+    blocksize : int, optional
+        If the blocksize is not degenerate with respect to the
+        size of the input array, then use a blocked algorithm. (Default: 64)
+
+    Returns
+    -------
+    sqrtm : (N, N) ndarray
+        Value of the sqrt function at `T`
+
+    References
+    ----------
+    .. [1] Edvin Deadman, Nicholas J. Higham, Rui Ralha (2013)
+           "Blocked Schur Algorithms for Computing the Matrix Square Root,
+           Lecture Notes in Computer Science, 7782. pp. 171-182.
+
+    """
+    T_diag = np.diag(T)
+    keep_it_real = np.isrealobj(T) and np.min(T_diag, initial=0.) >= 0
+
+    # Cast to complex as necessary + ensure double precision
+    if not keep_it_real:
+        T = np.asarray(T, dtype=np.complex128, order="C")
+        T_diag = np.asarray(T_diag, dtype=np.complex128)
+    else:
+        T = np.asarray(T, dtype=np.float64, order="C")
+        T_diag = np.asarray(T_diag, dtype=np.float64)
+
+    R = np.diag(np.sqrt(T_diag))
+
+    # Compute the number of blocks to use; use at least one block.
+    n, n = T.shape
+    nblocks = max(n // blocksize, 1)
+
+    # Compute the smaller of the two sizes of blocks that
+    # we will actually use, and compute the number of large blocks.
+    bsmall, nlarge = divmod(n, nblocks)
+    blarge = bsmall + 1
+    nsmall = nblocks - nlarge
+    if nsmall * bsmall + nlarge * blarge != n:
+        raise Exception('internal inconsistency')
+
+    # Define the index range covered by each block.
+    start_stop_pairs = []
+    start = 0
+    for count, size in ((nsmall, bsmall), (nlarge, blarge)):
+        for i in range(count):
+            start_stop_pairs.append((start, start + size))
+            start += size
+
+    # Within-block interactions (Cythonized)
+    try:
+        within_block_loop(R, T, start_stop_pairs, nblocks)
+    except RuntimeError as e:
+        raise SqrtmError(*e.args) from e
+
+    # Between-block interactions (Cython would give no significant speedup)
+    for j in range(nblocks):
+        jstart, jstop = start_stop_pairs[j]
+        for i in range(j-1, -1, -1):
+            istart, istop = start_stop_pairs[i]
+            S = T[istart:istop, jstart:jstop]
+            if j - i > 1:
+                S = S - R[istart:istop, istop:jstart].dot(R[istop:jstart,
+                                                            jstart:jstop])
+
+            # Invoke LAPACK.
+            # For more details, see the solve_sylvester implementation
+            # and the fortran dtrsyl and ztrsyl docs.
+            Rii = R[istart:istop, istart:istop]
+            Rjj = R[jstart:jstop, jstart:jstop]
+            if keep_it_real:
+                x, scale, info = dtrsyl(Rii, Rjj, S)
+            else:
+                x, scale, info = ztrsyl(Rii, Rjj, S)
+            R[istart:istop, jstart:jstop] = x * scale
+
+    # Return the matrix square root.
+    return R