# Source code for galgebra.metric

"""
Metric Tensor and Derivatives of Basis Vectors.
"""

import copy
import warnings
from typing import List, Optional

from sympy import (
diff, trigsimp, Matrix, Rational,
sqf_list, sqrt, eye, S, expand, Mul,
Add, simplify, Expr, Function, MatrixSymbol
)

from . import printer
from ._utils import cached_property as _cached_property
from .atoms import (
BasisVectorSymbol, DotProductSymbol, MatrixFunction, Determinant,
)

def apply_function_list(f, x):
if isinstance(f, (tuple, list)):
fx = x
for fi in f:
fx = fi(fx)
return fx
else:
return f(x)

[docs]def linear_expand(expr):
"""
linear_expand takes an expression that is the sum of a scalar
expression and a linear combination of noncommutative terms with
scalar coefficients and generates lists of coefficients and
noncommutative symbols the coefficients multiply.  The list of
noncommutatives symbols contains the scalar 1 if there is a scalar
term in the sum and also does not contain any repeated noncommutative
symbols.
"""
if not isinstance(expr, Expr):
raise TypeError('{!r} is not a SymPy Expr'.format(expr))

expr = expand(expr)

if expr == 0:
coefs = [expr]
bases = [S.One]
return (coefs, bases)

if isinstance(expr, Add):
args = expr.args
else:
if expr.is_commutative:
return ([expr], [S.One])
else:
args = [expr]
coefs = []
bases = []
for term in args:
if term.is_commutative:
if S.One in bases:
coefs[bases.index(S.One)] += term
else:
bases.append(S.One)
coefs.append(term)
else:
c, nc = term.args_cnc()
base = nc[0]
coef = Mul._from_args(c)
if base in bases:
coefs[bases.index(base)] += coef
else:
bases.append(base)
coefs.append(coef)
return (coefs, bases)

def linear_expand_terms(expr):
coefs, bases = linear_expand(expr)
return zip(coefs, bases)

[docs]def collect(A, nc_list):
"""
Parameters
-----------
A :
a linear combination of noncommutative symbols with scalar
expressions as coefficients
nc_list :
noncommutative symbols in A to combine

Returns
-------
sympy.Basic
A sum of the terms containing the noncommutative symbols in nc_list such that no elements
of nc_list appear more than once in the sum. All coefficients of a given element of nc_list
are combined into a single coefficient.
"""
coefs, bases = linear_expand(A)
C = S.Zero
for x in nc_list:
if x in bases:
i = bases.index(x)
bases.pop(i)
C += coefs.pop(i)*x

# add whatever is left
for c, b in zip(coefs, bases):
C += c * b

return C

[docs]def square_root_of_expr(expr):
"""
If expression is product of even powers then every power is divided
by two and the product is returned.  If some terms in product are
not even powers the sqrt of the absolute value of the expression is
returned.  If the expression is a number the sqrt of the absolute
value of the number is returned.
"""
if expr.is_number:
if expr > 0:
return sqrt(expr)
else:
return sqrt(-expr)
else:
expr = trigsimp(expr)
coef, pow_lst = sqf_list(expr)
if coef != S.One:
if coef.is_number:
coef = square_root_of_expr(coef)
else:
coef = sqrt(abs(coef))  # Product coefficient not a number
for p in pow_lst:
f, n = p
if n % 2 != 0:
return sqrt(abs(expr))  # Product not all even powers
else:
coef *= f ** (n / S(2))  # Positive sqrt of the square of an expression
return coef

[docs]def symbols_list(s, indices=None, sub=True, commutative=False):
"""
Convert a string to a list of symbols.

If :class:galgebra.printer.Eprint is enabled, the symbol names will
contain ANSI escape sequences.

Parameters
----------
s : str
Specification. If indices is specified, then this is just a prefix.
If indices is not specified then this is a string of one of the forms:

* prefix + "*" + index_1 + "|" + index_2 + "|" + ... + index_n
* prefix + "*" + n_indices
* name_1 + "," + name_2 + "," + ... + name_n
* name_1 + " " + name_2 + " " + ... + name_n

indices : list, optional
List of indices to append to the prefix.
sub : bool
If true, mark as subscript separating prefix and suffix with _, else
mark as superscript using __.
commutative : bool
Passed on to :class:sympy.Symbol.

Returns
-------
symbols : list of :class:sympy.Symbol

Examples
--------

Names can be comma or space separated:

>>> symbols_list('a,b,c')
[a, b, c]
>>> symbols_list('a b c')
[a, b, c]

Mixing commas and spaces gives surprising results:

>>> symbols_list('a b,c')
[a b, c]

A trailing comma is allowed, and required to generate lists of one element:

>>> symbols_list('a,')
[a]
>>> symbols_list('')
[]

Subscripts will be converted to superscripts if requested:

>>> symbols_list('a_1 a_2', sub=False)
[a__1, a__2]
>>> symbols_list('a__1 a__2', sub=False)
[a___1, a___2]

But not vice versa:

>>> symbols_list('a__1 a__2', sub=True)
[a__1, a__2]

Asterisk can be used for repetition:

>>> symbols_list('a*b|c|d')
[a_b, a_c, a_d]
>>> symbols_list('a*3')
[a_0, a_1, a_2]
>>> symbols_list('a*3')
[a_0, a_1, a_2]

Or the indices argument:

>>> symbols_list('a', [2, 4, 6])
[a_2, a_4, a_6]
>>> symbols_list('a', [2, 4, 6], sub=False)
[a__2, a__4, a__6]

See also
--------
:func:sympy.symbols: a similar function builtin to sympy
"""

if isinstance(s, list):  # s is already a list of symbols
return s

if sub is True:  # subscripted list
pos = '_'
else:  # superscripted list
pos = '__'

if indices is None:  # symbol list completely generated by s
if '*' in s:
[base, index] = s.split('*')
if '|' in s:
index = index.split('|')
s_lst = [base + pos + i for i in index]
else:  # symbol list indexed with integers 0 to n-1
try:
n = int(index)
except ValueError:
raise ValueError(index + 'is not an integer')
s_lst = [base + pos + str(i) for i in range(n)]
else:
if not s:
s_lst = []
elif ',' in s:
s_lst = s.split(',')
# allow trailing commas
if not s_lst[-1]:
del s_lst[-1]
else:
s_lst = s.split(' ')
if not sub:
s_lst = [x.replace('_', '__', 1) for x in s_lst]

else:  # indices symbol list used for sub/superscripts of generated symbol list
s_lst = [s + pos + str(i) for i in indices]
return [BasisVectorSymbol(s, commutative=commutative) for s in s_lst]

class Simp:
modes = [simplify]

@staticmethod
def profile(s):
Simp.modes = s

@staticmethod
def apply(expr):
obj = S.Zero
for coef, base in linear_expand_terms(expr):
obj += apply_function_list(Simp.modes, coef) * base
return obj

@staticmethod
def applymv(mv):
return Mv(Simp.apply(mv.obj), ga=mv.Ga)

[docs]class Metric(object):
"""
Metric specification

Attributes
----------

g : sympy matrix[,]
metric tensor
norm : list of sympy numbers
normalized diagonal metric tensor
coords : list[] of sympy symbols
coordinate variables
is_ortho : bool
True if basis is orthogonal
basis : list[] of non-commutative sympy variables
basis vector symbols
r_symbols : list[] of non-commutative sympy variables
reciprocal basis vector symbols
n : integer
dimension of vector space/manifold
n_range :
list of basis indices
de : list[][]
derivatives of basis functions.  Two dimensional list. First
entry is differentiating coordiate. Second entry is basis
vector.  Quantities are linear combinations of basis vector
symbols.
sig : Tuple[int, int]
Signature of metric (p,q) where n = p+q.  If metric tensor
is numerical and orthogonal it is calculated.  Otherwise the
following inputs are used:

=========   ===========  ==================================
Input       Signature     Type
=========   ===========  ==================================
"e"     (n,0)    Euclidean
"m+"    (n-1,1)  Minkowski (One negative square)
"m-"    (1,n-1)  Minkowski (One positive square)
p       (p,n-p)  General (integer not string input)
=========   ===========  ==================================

gsym : str
String for symbolic metric determinant.  If self.gsym = 'g'
then det(g) is sympy scalar function of coordinates with
name 'det(g)'.  Useful for complex non-orthogonal coordinate
systems or for calculations with general metric.
"""

count = 1

[docs]    @staticmethod
def dot_orthogonal(V1, V2, g=None):
"""
Returns the dot product of two vectors in an orthogonal coordinate
system.  V1 and V2 are lists of sympy expressions.  g is
a list of constants that gives the signature of the vector space to
allow for non-euclidian vector spaces.

This function is only used to form the dot product of vectors in the
embedding space of a vector manifold or in the case where the basis
vectors are explicitly defined by vector fields in the embedding
space.

A g of None is for a Euclidian embedding space.
"""
if g is None:
dot = 0
for v1, v2 in zip(V1, V2):
dot += v1 * v2
return dot
else:
if len(g) == len(V1):
dot = 0
for v1, v2, gii in zip(V1, V2, g):
dot += v1 * v2 * gii
return dot
else:
raise ValueError('In dot_orthogonal dimension of metric ' +
'must equal dimension of vector')

def _build_metric_element(self, s, i1, i2):
""" Build an element for the metric of bases[i1] . basis[i2] """
if s == '#':
if i1 <= i2:  # for default element ensure symmetry
return DotProductSymbol(self.basis[i1], self.basis[i2])
else:
return DotProductSymbol(self.basis[i2], self.basis[i1])
else:  # element is fraction or integer
return Rational(s)

[docs]    def metric_symbols_list(self, s=None):  # input metric tensor as string
"""
rows of metric tensor are separated by "," and elements
of each row separated by " ".  If the input is a single
row it is assummed that the metric tensor is diagonal.

Output is a square matrix.
"""
if s is None:
s = self.n * '# '
s = self.n * (s[:-1] + ',')
s = s[:-1]

if isinstance(s, str):
rows = s.split(',')
n_rows = len(rows)

if n_rows == 1:  # orthogonal metric
m_lst = s.split(' ')
m = [
self._build_metric_element(s, i, i)
for i, s in enumerate(m_lst)
]

if len(m) != self.n:
raise ValueError('Input metric "' + s + '" has' +
' different rank than bases "' + str(self.basis) + '"')
diagonal = eye(self.n)

for i in self.n_range:
diagonal[i, i] = m[i]
return diagonal

else:  # non orthogonal metric
rows = s.split(',')
n_rows = len(rows)
m_lst = []
for row in rows:
cols = row.strip().split(' ')
n_cols = len(cols)
if n_rows != n_cols:  # non square metric
raise ValueError("'" + s + "' does not represent square metric")
m_lst.append(cols)
n = len(m_lst)
if n != self.n:
raise ValueError('Input metric "' + s + '" has' +
' different rank than bases "' + str(self.basis) + '"')
return Matrix([
[
self._build_metric_element(s, i1, i2)
for i2, s in enumerate(row)
]
for i1, row in enumerate(m_lst)
])

def derivatives_of_g(self):
# galgebra 0.5.0
warnings.warn(
"Metric.derivatives_of_g is deprecated, and now does nothing. "
"the .dg property is now always available.",
DeprecationWarning, stacklevel=2)

@_cached_property
def dg(self) -> List[List[List[Expr]]]:
# dg[i][j][k] = \partial_{x_{k}}g_{ij}
return [[[
diff(self.g[i, j], x_k)
for x_k in self.coords]
for j in self.n_range]
for i in self.n_range]

@_cached_property
def connect_flg(self) -> bool:
""" True if connection is non-zero """
if self.coords is None:
return False
else:
return any(
self.dg[i][j][k] != 0
for i in self.n_range
for j in self.n_range
for k in self.n_range
)

@_cached_property
def de(self) -> Optional[List[List[Expr]]]:
# Derivatives of basis vectors from Christoffel symbols

n_range = self.n_range

if not self.connect_flg:
return None

# Christoffel symbols of the first kind, \Gamma_{ijk}
# TODO handle None
dG = self.Christoffel_symbols(mode=1)

# de[i][j] = \partial_{x_{i}}e^{x_{j}}
# \frac{\partial e_{j}}{\partial x^{i}} = \Gamma_{ijk} e^{k}
de = [[
sum([Gamma_ijk * e__k for Gamma_ijk, e__k in zip(dG[i][j], self.r_symbols)])
for j in n_range
] for i in n_range]

if self.debug:
printer.oprint('D_{i}e^{j}', de)

return de

def inverse_metric(self) -> None:
# galgebra 0.5.0
warnings.warn(
"Metric.inverse_metric is deprecated, and now does nothing. "
"the .g_inv property is now always available.",
DeprecationWarning, stacklevel=2)

@_cached_property
def g_inv(self) -> Matrix:
""" Inverse of metric tensor :attr:g """
if self.is_ortho:  # Orthogonal metric
g_inv = eye(self.n)
for i in range(self.n):
g_inv[i, i] = S.One/self.g(i, i)
return g_inv
elif self.gsym is None:
return simplify(self.g.inv())
else:
return self.g_adj/self.detg

@_cached_property
def g_adj(self) -> Matrix:
""" Adjugate of g """
return simplify(self.g.adjugate())

[docs]    def Christoffel_symbols(self, mode=1):
"""
mode = 1  Christoffel symbols of the first kind
mode = 2  Christoffel symbols of the second kind
"""

# See if connection is zero
if not self.connect_flg:
return

n_range = self.n_range

# dg[i][j][k] = \partial_{x_{k}}g_{ij}
dg = self.dg

if mode == 1:

# Christoffel symbols of the first kind, \Gamma_{ijk}
# \partial_{x^{i}}e_{j} = \Gamma_{ijk}e^{k}

def Gamma_ijk(i, j, k):
return S.Half * (dg[j][k][i] + dg[i][k][j] - dg[i][j][k])

# dG[i][j][k] = S.Half * (dg[j][k][i] + dg[i][k][j] - dg[i][j][k])
dG = [[[
Simp.apply(Gamma_ijk(i, j, k))
for k in n_range]
for j in n_range]
for i in n_range]

if self.debug:
printer.oprint('Gamma_{ijk}', dG)
return dG

elif mode == 2:
# TODO handle None
Gamma1 = self.Christoffel_symbols(mode=1)

# Christoffel symbols of the second kind, \Gamma_{ij}^{k} = \Gamma_{ijl}g^{lk}
# \partial_{x^{i}}e_{j} = \Gamma_{ij}^{k}e_{k}

def Gamma2_ijk(i, j, k):
return sum([Gamma_ijl * self.g_inv[l, k] for l, Gamma_ijl in enumerate(Gamma1[i][j])])

Gamma2 = [[[
Simp.apply(Gamma2_ijk(i, j, k))
for k in n_range]
for j in n_range]
for i in n_range]

return Gamma2
else:
raise ValueError('In Christoffle_symobols mode = ' + str(mode) + ' is not allowed\n')

def normalize_metric(self):

# normalize derivatives
if self.de is not None:
# Generate mapping for renormalizing reciprocal basis vectors
renorm = [
(self.r_symbols[ib], self.r_symbols[ib] / self.e_norm[ib])
for ib in self.n_range  # e^{ib} --> e^{ib}/|e_{ib}|
]

# Normalize derivatives of basis vectors
for x_i in self.n_range:
for jb in self.n_range:
self.de[x_i][jb] = Simp.apply((((self.de[x_i][jb].subs(renorm)
- diff(self.e_norm[jb], self.coords[x_i]) *
self.basis[jb]) / self.e_norm[jb])))
if self.debug:
printer.oprint('e^{i}->e^{i}/|e_{i}|', renorm)
for x_i in self.n_range:
for jb in self.n_range:
print(r'\partial_{' + str(self.coords[x_i]) + r'}\hat{e}_{' + str(self.coords[jb]) + '} =', self.de[x_i][jb])

# Normalize metric tensor
for ib in self.n_range:
for jb in self.n_range:
self.g[ib, jb] = Simp.apply(self.g[ib, jb] / (self.e_norm[ib] * self.e_norm[jb]))

if self.debug:
printer.oprint('renorm(g)', self.g)

def signature(self):
if self.is_ortho:
p = 0
q = 0
for i in self.n_range:
g_ii = self.g[i, i]
if g_ii.is_number:
if g_ii > 0:
p += 1
else:
q += 1
else:
break
if p + q == self.n:
self.sig = (p, q)
return
if isinstance(self.sig, int):  # General signature
if self.sig <= self.n:
self.sig = (self.sig, self.n - self.sig)
return
else:
raise ValueError('self.sig = ' + str(self.sig) + ' > self.n, not an allowed hint')
if isinstance(self.sig, str):
if self.sig == 'e':  # Euclidean metric signature
self.sig = (self.n, 0)
elif self.sig == 'm+':  # Minkowski metric signature (n-1,1)
self.sig = (self.n - 1, 1)
elif self.sig == 'm-':  # Minkowski metric signature (1,n-1)
self.sig = (1, self.n - 1)
else:
raise ValueError('self.sig = ' + str(self.sig) + ' is not an allowed hint')
return
raise ValueError(str(self.sig) + ' is not allowed value for self.sig')

@_cached_property
def detg(self) -> Expr:
r""" Determinant of :math:g, :math:\det g """
if self.gsym is None:
g = self.g
else:
# Define name of metric tensor determinant as sympy symbol
if self.coords is None:
g = MatrixSymbol(self.gsym, self.n, self.n)
else:
g = MatrixFunction(self.gsym, self.n, self.n)(*self.coords)
return Determinant(g)

def __init__(
self, basis, *,
g=None,
coords=None,
X=None,
norm=False,
debug=False,
gsym=None,
sig='e',
Isq='-'
):
"""
Parameters
----------
basis :
string specification
g :
metric tensor
coords :
manifold/vector space coordinate list/tuple  (sympy symbols)
X :
vector manifold function
norm :
True to normalize basis vectors
debug :
True to print out debugging information
gsym :
String s to use "det("+s+")" function in reciprocal basis
sig :
Signature of metric, default is (n,0) a Euclidean metric
Isq :
Sign of square of pseudo-scalar, default is '-'
"""

self.name = 'GA' + str(Metric.count)
Metric.count += 1

if not isinstance(basis, str):
raise TypeError('"' + str(basis) + '" must be string')

self.sig = sig  # Hint for metric signature
self.gsym = gsym
self.Isq = Isq  #: Sign of I**2, only needed if I**2 not a number

self.debug = debug
self.is_ortho = False  # Is basis othogonal
self.coords = coords  # Manifold coordinates
self.norm = norm  # True to normalize basis vectors
# Generate list of basis vectors and reciprocal basis vectors
# as non-commutative symbols

if ' ' in basis or ',' in basis or '*' in basis or basis == '':  # bases defined by substrings separated by spaces or commas
self.basis = symbols_list(basis)
self.r_symbols = symbols_list(basis, sub=False)
else:
if coords is not None:  # basis defined by root string with symbol list as indices
self.basis = symbols_list(basis, coords)
self.r_symbols = symbols_list(basis, coords, sub=False)
self.coords = coords
if self.debug:
printer.oprint('x^{i}', self.coords)
else:
raise ValueError('for basis "' + basis + '" coords must be entered')

if self.debug:
printer.oprint('e_{i}', self.basis, 'e^{i}', self.r_symbols)
self.n = len(self.basis)
self.n_range = list(range(self.n))

# Generate metric as list of lists of symbols, rationals, or functions of coordinates

if g is None:
if X is None:  # default metric from dot product of basis as symbols
self.g = self.metric_symbols_list()
else:  # Vector manifold
if coords is None:
raise ValueError('For metric derived from vector field ' +
' coordinates must be defined.')
else:  # Vector manifold defined by vector field
# Get basis vectors by differentiating vector field
dX = [
[diff(x, coord) for x in X]
for coord in coords
]
self.g = Matrix([
[
trigsimp(Metric.dot_orthogonal(dx1, dx2, g))
for dx2 in dX
]
for dx1 in dX
])
if self.debug:
printer.oprint('X_{i}', X, 'D_{i}X_{j}', dX)

else:  # metric is symbolic or list of lists of functions of coordinates
if isinstance(g, str):  # metric elements are symbols or constants
if g == 'g':  # general symbolic metric tensor (g_ij functions of position)
self.g = Matrix([
[
Function('g_{}_{}'.format(coord, coord2))(*self.coords)
for coord2 in self.coords
]
for coord in self.coords
])
self.g_inv = Matrix([
[
Function('g__{}__{}'.format(coord, coord2))(*self.coords)
for coord2 in self.coords
]
for coord in self.coords
])
else:  # specific symbolic metric tensor (g_ij are symbolic or numerical constants)
self.g = self.metric_symbols_list(g)  # construct symbolic metric from string and basis
else:  # metric is given as list of function or list of lists of function or matrix of functions
if isinstance(g, Matrix):
self.g = g
else:
if len(g) > 0 and isinstance(g[0], list):
self.g = Matrix(g)
else:
m = eye(len(g))
for i in range(len(g)):
m[i, i] = g[i]
self.g = m

self.g_raw = copy.deepcopy(self.g)  # save original metric tensor for use with submanifolds

if self.debug:
printer.oprint('g', self.g)

# Determine if metric is orthogonal

self.is_ortho = all(
self.g[i, j] == 0
for i in self.n_range
for j in self.n_range
if i < j
)

self.g_is_numeric = all(
self.g[i, j].is_number
for i in self.n_range
for j in self.n_range
if i < j
)

if self.norm:  # normalize basis, metric, and derivatives of normalized basis
if not self.is_ortho:
raise ValueError('!!!!Basis normalization only implemented for orthogonal basis!!!!')
self.e_norm = [
square_root_of_expr(self.g[i, i])
for i in self.n_range
]
if debug:
printer.oprint('|e_{i}|', self.e_norm)
self.normalize_metric()
else:
self.e_norm = None

if not self.g_is_numeric:
self.signature()
# Sign of square of pseudo scalar
self.e_sq_sgn = '+'
if ((self.n*(self.n-1))//2+self.sig[1]) % 2 == 1:
self.e_sq_sgn = '-'

if self.debug:
print('signature =', self.sig)