Source code for sympy.series.order

from __future__ import print_function, division

from sympy.core import Basic, S, sympify, Expr, Rational, Symbol
from sympy.core import Add, Mul, expand_power_base, expand_log
from sympy.core.cache import cacheit
from sympy.core.compatibility import default_sort_key, is_sequence
from sympy.core.containers import Tuple
from sympy.utilities.iterables import uniq


[docs]class Order(Expr): r""" 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 all limit points to be 0 or positive infinity 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} \sup |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, oo >>> from sympy.abc import x, y >>> O(x + x**2) O(x) >>> O(x + x**2, (x, 0)) O(x) >>> O(x + x**2, (x, oo)) O(x**2, (x, oo)) >>> O(1 + x*y) O(1, x, y) >>> O(1 + x*y, (x, 0), (y, 0)) O(1, x, y) >>> O(1 + x*y, (x, oo), (y, oo)) O(x*y, (x, oo), (y, oo)) >>> O(1) in O(1, x) True >>> O(1, x) in O(1) False >>> O(x) in O(1, x) True >>> O(x**2) in O(x) True >>> 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, *args, **kwargs): expr = sympify(expr) if not args: if expr.is_Order: variables = expr.variables point = expr.point else: variables = list(expr.free_symbols) point = [S.Zero]*len(variables) else: args = list(args if is_sequence(args) else [args]) variables, point = [], [] if is_sequence(args[0]): for a in args: v, p = list(map(sympify, a)) variables.append(v) point.append(p) else: variables = list(map(sympify, args)) point = [S.Zero]*len(variables) if not all(isinstance(v, Symbol) for v in variables): raise TypeError('Variables are not symbols, got %s' % variables) if len(list(uniq(variables))) != len(variables): raise ValueError('Variables are supposed to be unique symbols, got %s' % variables) if expr.is_Order: expr_vp = dict(expr.args[1:]) new_vp = dict(expr_vp) vp = dict(zip(variables, point)) for v, p in vp.items(): if v in new_vp.keys(): if p != new_vp[v]: raise NotImplementedError( "Mixing Order at different points is not supported.") else: new_vp[v] = p if set(expr_vp.keys()) == set(new_vp.keys()): return expr else: variables = list(new_vp.keys()) point = [new_vp[v] for v in variables] if expr is S.NaN: return S.NaN if not all(p is S.Zero for p in point) and \ not all(p is S.Infinity for p in point): raise NotImplementedError('Order at points other than 0 ' 'or oo not supported, got %s as a point.' % point) if variables: if len(variables) > 1: # XXX: better way? We need this expand() to # workaround e.g: expr = x*(x + y). # (x*(x + y)).as_leading_term(x, y) currently returns # x*y (wrong order term!). That's why we want to deal with # expand()'ed expr (handled in "if expr.is_Add" branch below). expr = expr.expand() if expr.is_Add: lst = expr.extract_leading_order(variables, point) expr = Add(*[f.expr for (e, f) in lst]) elif expr: if point[0] == S.Zero: expr = expr.as_leading_term(*variables) expr = expr.as_independent(*variables, as_Add=False)[1] expr = expand_power_base(expr) expr = expand_log(expr) if len(variables) == 1: # The definition of O(f(x)) symbol explicitly stated that # the argument of f(x) is irrelevant. That's why we can # combine some power exponents (only "on top" of the # expression tree for f(x)), e.g.: # x**p * (-x)**q -> x**(p+q) for real p, q. x = variables[0] margs = list(Mul.make_args( expr.as_independent(x, as_Add=False)[1])) for i, t in enumerate(margs): if t.is_Pow: b, q = t.args if b in (x, -x) and q.is_real and not q.has(x): margs[i] = x**q elif b.is_Pow and not b.exp.has(x): b, r = b.args if b in (x, -x) and r.is_real: margs[i] = x**(r*q) elif b.is_Mul and b.args[0] is S.NegativeOne: b = -b if b.is_Pow and not b.exp.has(x): b, r = b.args if b in (x, -x) and r.is_real: margs[i] = x**(r*q) expr = Mul(*margs) if expr is S.Zero: return expr if expr.is_Order: expr = expr.expr if not expr.has(*variables): expr = S.One # create Order instance: variables.sort(key=default_sort_key) args = (expr,) + Tuple(*zip(variables, point)) obj = Expr.__new__(cls, *args) 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): if self.args[1:]: return tuple(x[0] for x in self.args[1:]) else: return () @property def point(self): if self.args[1:]: return tuple(x[1] for x in self.args[1:]) else: return () @property def free_symbols(self): return self.expr.free_symbols | set(self.variables) def _eval_power(b, e): if e.is_Number and e.is_nonnegative: return b.func(b.expr ** e, *b.args[1:]) return def as_expr_variables(self, order_symbols): if order_symbols is None: order_symbols = self.args[1:] else: if not all(o[1] == order_symbols[0][1] for o in order_symbols) and \ not all(p == self.point[0] for p in self.point): raise NotImplementedError('Order at points other than 0 ' 'or oo not supported, got %s as a point.' % point) if order_symbols[0][1] != self.point[0]: raise NotImplementedError( "Multiplying Order at different points is not supported.") order_symbols = dict(order_symbols) for s, p in dict(self.args[1:]).items(): if s not in order_symbols.keys(): order_symbols[s] = p order_symbols = sorted(order_symbols.items(), key=lambda x: default_sort_key(x[0])) return self.expr, tuple(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). """ from sympy import powsimp, limit if expr is S.Zero: return True if expr is S.NaN: return False if expr.is_Order: if not all(p == expr.point[0] for p in expr.point) and \ not all(p == self.point[0] for p in self.point): raise NotImplementedError('Order at points other than 0 ' 'or oo not supported, got %s as a point.' % point) else: # self and/or expr is O(1): if any(not p for p in [expr.point, self.point]): point = self.point + expr.point if point: point = point[0] else: point = S.Zero else: point = self.point[0] if expr.expr == self.expr: # O(1) + O(1), O(1) + O(1, x), etc. return all([x in self.args[1:] for x in expr.args[1:]]) if expr.expr.is_Add: return all([self.contains(x) for x in expr.expr.args]) if self.expr.is_Add: return any([self.func(x, *self.args[1:]).contains(expr) for x in self.expr.args]) 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: return None r = None ratio = self.expr/expr.expr ratio = powsimp(ratio, deep=True, combine='exp') for s in common_symbols: l = limit(ratio, s, point) != 0 if r is None: r = l else: if r != l: return return r obj = self.func(expr, *self.args[1:]) return self.contains(obj)
def __contains__(self, other): result = self.contains(other) if result is None: raise TypeError('contains did not evaluate to a bool') return result def _eval_subs(self, old, new): if old.is_Symbol and old in self.variables: i = self.variables.index(old) newexpr = self.expr._subs(old, new) if isinstance(new, Symbol): newvars = list(self.variables) newvars[i] = new newpt = self.point else: newvars = tuple(newexpr.free_symbols) + \ self.variables[:i] + self.variables[i + 1:] newpt = self.point[0]**(new.as_numer_denom()[1].is_number*2 - 1) newpt = [newpt]*len(newvars) return Order(newexpr, *zip(newvars, newpt)) return Order(self.expr._subs(old, new), *self.args[1:]) def _eval_conjugate(self): expr = self.expr._eval_conjugate() if expr is not None: return self.func(expr, *self.args[1:]) def _eval_derivative(self, x): return self.func(self.expr.diff(x), *self.args[1:]) or self def _eval_transpose(self): expr = self.expr._eval_transpose() if expr is not None: return self.func(expr, *self.args[1:]) def _sage_(self): #XXX: SAGE doesn't have Order yet. Let's return 0 instead. return Rational(0)._sage_()
O = Order