Source code for sympy.series.order

from sympy.core import Basic, S, sympify, Expr, Rational, Symbol
from sympy.core.cache import cacheit
from sympy.core.compatibility import cmp_to_key

[docs]class Order(Expr):
""" Represents the limiting behavior of some function

The order of a function characterizes the function based on the limiting
behavior of the function as it goes to some limit. Only taking the limit
point to be 0 is currently supported. This is expressed in big O notation
[1]_.

The formal definition for the order of a function g(x) about a point a
is such that g(x) = O(f(x)) as x \\rightarrow a if and only if for any
\delta > 0 there exists a M > 0 such that |g(x)| \leq M|f(x)| for
|x-a| < \delta. This is equivalent to \lim_{x \\rightarrow a}
|g(x)/f(x)| < \infty.

Let's illustrate it on the following example by taking the expansion of
\sin(x) about 0:

.. math ::
\sin(x) = x - x^3/3! + O(x^5)

where in this case O(x^5) = x^5/5! - x^7/7! + \cdots. By the definition
of O, for any \delta > 0 there is an M such that:

.. math ::
|x^5/5! - x^7/7! + ....| <= M|x^5| \\text{ for } |x| < \delta

or by the alternate definition:

.. math ::
\lim_{x \\rightarrow 0} | (x^5/5! - x^7/7! + ....) / x^5| < \infty

which surely is true, because

.. math ::
\lim_{x \\rightarrow 0} | (x^5/5! - x^7/7! + ....) / x^5| = 1/5!

As it is usually used, the order of a function can be intuitively thought
of representing all terms of powers greater than the one specified. For
example, O(x^3) corresponds to any terms proportional to x^3,
x^4,\ldots and any higher power. For a polynomial, this leaves terms
proportional to x^2, x and constants.

Examples
========

>>> from sympy import O
>>> from sympy.abc import x
>>> O(x)
O(x)
>>> O(x)*x
O(x**2)
>>> O(x)-O(x)
O(x)

References
==========

.. [1] Big O notation <http://en.wikipedia.org/wiki/Big_O_notation>_

Notes
=====

In O(f(x), x) the expression f(x) is assumed to have a leading
term.  O(f(x), x) is automatically transformed to
O(f(x).as_leading_term(x),x).

O(expr*f(x), x) is O(f(x), x)

O(expr, x) is O(1)

O(0, x) is 0.

Multivariate O is also supported:

O(f(x, y), x, y) is transformed to
O(f(x, y).as_leading_term(x,y).as_leading_term(y), x, y)

In the multivariate case, it is assumed the limits w.r.t. the various
symbols commute.

If no symbols are passed then all symbols in the expression are used:

"""

is_Order = True

__slots__ = []

@cacheit
def __new__(cls, expr, *symbols, **assumptions):

expr = sympify(expr).expand()
if expr is S.NaN:
return S.NaN

if symbols:
symbols = map(sympify, symbols)
if not all(isinstance(s, Symbol) for s in symbols):
raise NotImplementedError('Order at points other than 0 not supported.')
else:
symbols = list(expr.free_symbols)

if expr.is_Order:

new_symbols = list(expr.variables)
for s in symbols:
if s not in new_symbols:
new_symbols.append(s)
if len(new_symbols) == len(expr.variables):
return expr
symbols = new_symbols

elif symbols:

expr = Add(*[f.expr for (e,f) in lst])
elif expr:
if len(symbols) > 1:
# TODO
# We cannot use compute_leading_term because that only
# works in one symbol.
else:
terms = expr.as_coeff_mul(*symbols)[1]
s = set(symbols)
expr = Mul(*[t for t in terms if s & t.free_symbols])

if expr is S.Zero:
return expr
elif not expr.has(*symbols):
expr = S.One

# create Order instance:
symbols.sort(key=cmp_to_key(Basic.compare))
obj = Expr.__new__(cls, expr, *symbols, **assumptions)

return obj

def _hashable_content(self):
return self.args

def oseries(self, order):
return self

def _eval_nseries(self, x, n, logx):
return self

@property
def expr(self):
return self._args[0]

@property
def variables(self):
return self._args[1:]

@property
def free_symbols(self):
return self.expr.free_symbols

def _eval_power(b, e):
if e.is_Number:
return Order(b.expr ** e, *b.variables)
return

def as_expr_variables(self, order_symbols):
if order_symbols is None:
order_symbols = self.variables
else:
for s in self.variables:
if s not in order_symbols:
order_symbols = order_symbols + (s,)
return self.expr, order_symbols

def removeO(self):
return S.Zero

def getO(self):
return self

@cacheit
[docs]    def contains(self, expr):
"""
Return True if expr belongs to Order(self.expr, \*self.variables).
Return False if self belongs to expr.
Return None if the inclusion relation cannot be determined
(e.g. when self and expr have different symbols).
"""
# NOTE: when multiplying out series a lot of queries like
#       O(...).contains(a*x**b) with many a and few b are made.
#       Separating out the independent part allows for better caching.
c, m = expr.as_coeff_mul(*self.variables)
if m != ():
return self._contains(Mul(*m))
else:
# Mul(*m) == 1, and O(1) treatment is somewhat peculiar ...
# some day this else should not be necessary
return self._contains(expr)

@cacheit
def _contains(self, expr):
from sympy import powsimp, limit
if expr is S.Zero:
return True
if expr is S.NaN:
return False
if expr.is_Order:
if self.variables and expr.variables:
common_symbols = tuple([s for s in self.variables if s in expr.variables])
elif self.variables:
common_symbols = self.variables
else:
common_symbols = expr.variables
if not common_symbols:
if not (self.variables or expr.variables): # O(1),O(1)
return True
return None
r = None
for s in common_symbols:
l = limit(powsimp(self.expr/expr.expr, deep=True,\
combine='exp'), s, 0) != 0
if r is None:
r = l
else:
if r != l:
return
return r
obj = Order(expr, *self.variables)
return self.contains(obj)

def _eval_subs(self, old, new):
if old.is_Symbol and old in self.variables:
i = list(self.variables).index(old)
if isinstance(new, Symbol):
return Order(self.expr._subs(old, new), *(self.variables[:i]+(new,)+self.variables[i+1:]))
return Order(self.expr._subs(old, new), *(self.variables[:i]+self.variables[i+1:]))
return Order(self.expr._subs(old, new), *self.variables)

def _eval_derivative(self, x):
return self.func(self.expr.diff(x), *self.variables) or self

def _sage_(self):
#XXX: SAGE doesn't have Order yet. Let's return 0 instead.
return Rational(0)._sage_()

O = Order