Symbolic Series

Symbolic series are special kinds of symbolic expressions that are constructed via the Expression.series method. They usually have an Order() term unless the series representation is exact, see is_terminating_series().

For series over general rings see power series and Laurent series.

EXAMPLES:

We expand a polynomial in \(x\) about \(0\), about \(1\), and also truncate it back to a polynomial:

sage: var('x,y')
(x, y)
sage: f = (x^3 - sin(y)*x^2 - 5*x + 3); f
x^3 - x^2*sin(y) - 5*x + 3
sage: g = f.series(x, 4); g
3 + (-5)*x + (-sin(y))*x^2 + 1*x^3 + Order(x^4)
sage: g.truncate()
x^3 - x^2*sin(y) - 5*x + 3
sage: g = f.series(x==1, oo); g
(-sin(y) - 1) + (-2*sin(y) - 2)*(x - 1) + (-sin(y) + 3)*(x - 1)^2 + 1*(x - 1)^3
sage: h = g.truncate(); h
(x - 1)^3 - (x - 1)^2*(sin(y) - 3) - 2*(x - 1)*(sin(y) + 1) - sin(y) - 1
sage: h.expand()
x^3 - x^2*sin(y) - 5*x + 3

We compute another series expansion of an analytic function:

sage: f = sin(x)/x^2
sage: f.series(x,7)
1*x^(-1) + (-1/6)*x + 1/120*x^3 + (-1/5040)*x^5 + Order(x^7)
sage: f.series(x==1,3)
(sin(1)) + (cos(1) - 2*sin(1))*(x - 1) + (-2*cos(1) + 5/2*sin(1))*(x - 1)^2 + Order((x - 1)^3)
sage: f.series(x==1,3).truncate().expand()
-2*x^2*cos(1) + 5/2*x^2*sin(1) + 5*x*cos(1) - 7*x*sin(1) - 3*cos(1) + 11/2*sin(1)

Following the GiNaC tutorial, we use John Machin’s amazing formula \(\pi = 16 \mathrm{tan}^{-1}(1/5) - 4 \mathrm{tan}^{-1}(1/239)\) to compute digits of \(\pi\). We expand the arc tangent around 0 and insert the fractions 1/5 and 1/239.

sage: x = var('x')
sage: f = atan(x).series(x, 10); f
1*x + (-1/3)*x^3 + 1/5*x^5 + (-1/7)*x^7 + 1/9*x^9 + Order(x^10)
sage: (16*f.subs(x==1/5) - 4*f.subs(x==1/239)).n()
3.14159268240440

Note: The result of an operation or function of series is not automatically expanded to a series. This must be explicitly done by the user:

sage: ex1 = sin(x).series(x, 4); ex1
1*x + (-1/6)*x^3 + Order(x^4)
sage: ex2 = cos(x).series(x, 4); ex2
1 + (-1/2)*x^2 + Order(x^4)
sage: ex1 + ex2
(1 + (-1/2)*x^2 + Order(x^4)) + (1*x + (-1/6)*x^3 + Order(x^4))
sage: (ex1 + ex2).series(x,4)
1 + 1*x + (-1/2)*x^2 + (-1/6)*x^3 + Order(x^4)
sage: x*ex1
x*(1*x + (-1/6)*x^3 + Order(x^4))
sage: (x*ex1).series(x,5)
1*x^2 + (-1/6)*x^4 + Order(x^5)
sage: sin(ex1)
sin(1*x + (-1/6)*x^3 + Order(x^4))
sage: sin(ex1).series(x,9)
1*x + (-1/3)*x^3 + 11/120*x^5 + (-53/2520)*x^7 + Order(x^9)
sage: (sin(x^2)^(-5)).series(x,3)
1*x^(-10) + 5/6*x^(-6) + 3/8*x^(-2) + 367/3024*x^2 + Order(x^3)
sage: (cot(x)^(-3)).series(x,3)
Order(x^3)
sage: (cot(x)^(-3)).series(x,4)
1*x^3 + Order(x^4)

class sage.symbolic.series.SymbolicSeries

Bases: sage.symbolic.expression.Expression

Trivial constructor.

EXAMPLES:

sage: loads(dumps((x+x^3).series(x,2)))
1*x + Order(x^2)

coefficients(x=None, sparse=True)

Return the coefficients of this symbolic series as a list of pairs.

INPUT:

• x – optional variable.

• sparse – Boolean. If False return a list with as much

  entries as the order of the series.

OUTPUT:

Depending on the value of sparse,

• A list of pairs (expr, n), where expr is a symbolic expression and n is a power (sparse=True, default)

• A list of expressions where the n-th element is the coefficient of x^n when self is seen as polynomial in x (sparse=False).

EXAMPLES:

sage: s=(1/(1-x)).series(x,6); s
1 + 1*x + 1*x^2 + 1*x^3 + 1*x^4 + 1*x^5 + Order(x^6)
sage: s.coefficients()
[[1, 0], [1, 1], [1, 2], [1, 3], [1, 4], [1, 5]]
sage: s.coefficients(x, sparse=False)
[1, 1, 1, 1, 1, 1]
sage: x,y = var("x,y")
sage: s=(1/(1-y*x-x)).series(x,3); s
1 + (y + 1)*x + ((y + 1)^2)*x^2 + Order(x^3)
sage: s.coefficients(x, sparse=False)
[1, y + 1, (y + 1)^2]

default_variable()

Return the expansion variable of this symbolic series.

EXAMPLES:

sage: s=(1/(1-x)).series(x,3); s
1 + 1*x + 1*x^2 + Order(x^3)
sage: s.default_variable()
x

is_terminating_series()

Return True if the series is without order term.

A series is terminating if it can be represented exactly, without requiring an order term. You can explicitly request terminating series by setting the order to positive infinity.

OUTPUT:

Boolean. True if the series has no order term.

EXAMPLES:

sage: (x^5+x^2+1).series(x, +oo)
1 + 1*x^2 + 1*x^5
sage: (x^5+x^2+1).series(x,+oo).is_terminating_series()
True
sage: SR(5).is_terminating_series()
False
sage: exp(x).series(x,10).is_terminating_series()
False

power_series(base_ring)

Return algebraic power series associated to this symbolic series. The coefficients must be coercible to the base ring.

EXAMPLES:

sage: ex=(gamma(1-x)).series(x,3); ex
1 + euler_gamma*x + (1/2*euler_gamma^2 + 1/12*pi^2)*x^2 + Order(x^3)
sage: g=ex.power_series(SR); g
1 + euler_gamma*x + (1/2*euler_gamma^2 + 1/12*pi^2)*x^2 + O(x^3)
sage: g.parent()
Power Series Ring in x over Symbolic Ring

truncate()

Given a power series or expression, return the corresponding expression without the big oh.

OUTPUT:

A symbolic expression.

EXAMPLES:

sage: f = sin(x)/x^2
sage: f.truncate()
sin(x)/x^2
sage: f.series(x,7)
1*x^(-1) + (-1/6)*x + 1/120*x^3 + (-1/5040)*x^5 + Order(x^7)
sage: f.series(x,7).truncate()
-1/5040*x^5 + 1/120*x^3 - 1/6*x + 1/x
sage: f.series(x==1,3).truncate().expand()
-2*x^2*cos(1) + 5/2*x^2*sin(1) + 5*x*cos(1) - 7*x*sin(1) - 3*cos(1) + 11/2*sin(1)