Multivariate Power Series

Construct and manipulate multivariate power series (in finitely many variables) over a given commutative ring. Multivariate power series are implemented with total-degree precision.

EXAMPLES:

Power series arithmetic, tracking precision:

sage: R.<s,t> = PowerSeriesRing(ZZ); R
Multivariate Power Series Ring in s, t over Integer Ring

sage: f = 1 + s + 3*s^2; f
1 + s + 3*s^2
sage: g = t^2*s + 3*t^2*s^2 + R.O(5); g
s*t^2 + 3*s^2*t^2 + O(s, t)^5
sage: g = t^2*s + 3*t^2*s^2 + O(s, t)^5; g
s*t^2 + 3*s^2*t^2 + O(s, t)^5
sage: f = f.O(7); f
1 + s + 3*s^2 + O(s, t)^7
sage: f += s; f
1 + 2*s + 3*s^2 + O(s, t)^7
sage: f*g
s*t^2 + 5*s^2*t^2 + O(s, t)^5
sage: (f-1)*g
2*s^2*t^2 + 9*s^3*t^2 + O(s, t)^6
sage: f*g - g
2*s^2*t^2 + O(s, t)^5

sage: f*=s; f
s + 2*s^2 + 3*s^3 + O(s, t)^8
sage: f%2
s + s^3 + O(s, t)^8
sage: (f%2).parent()
Multivariate Power Series Ring in s, t over Ring of integers modulo 2

As with univariate power series, comparison of \(f\) and \(g\) is done up to the minimum precision of \(f\) and \(g\):

sage: f = 1 + t + s + s*t + R.O(3); f
1 + s + t + s*t + O(s, t)^3
sage: g = s^2 + 2*s^4 - s^5 + s^2*t^3 + R.O(6); g
s^2 + 2*s^4 - s^5 + s^2*t^3 + O(s, t)^6
sage: f == g
False
sage: g == g.add_bigoh(3)
True
sage: f < g
False
sage: f > g
True

Calling:

sage: f = s^2 + s*t + s^3 + s^2*t + 3*s^4 + 3*s^3*t + R.O(5); f
s^2 + s*t + s^3 + s^2*t + 3*s^4 + 3*s^3*t + O(s, t)^5
sage: f(t,s)
s*t + t^2 + s*t^2 + t^3 + 3*s*t^3 + 3*t^4 + O(s, t)^5
sage: f(t^2,s^2)
s^2*t^2 + t^4 + s^2*t^4 + t^6 + 3*s^2*t^6 + 3*t^8 + O(s, t)^10

Substitution is defined only for elements of positive valuation, unless \(f\) has infinite precision:

sage: f(t^2,s^2+1)
Traceback (most recent call last):
...
TypeError: Substitution defined only for elements of positive valuation,
unless self has infinite precision.

sage: g = f.truncate()
sage: g(t^2,s^2+1)
t^2 + s^2*t^2 + 2*t^4 + s^2*t^4 + 4*t^6 + 3*s^2*t^6 + 3*t^8
sage: g(t^2,(s^2+1).O(3))
t^2 + s^2*t^2 + 2*t^4 + O(s, t)^5

0 has valuation +Infinity:

sage: f(t^2,0)
t^4 + t^6 + 3*t^8 + O(s, t)^10
sage: f(t^2,s^2+s)
s*t^2 + s^2*t^2 + t^4 + O(s, t)^5

Substitution of power series with finite precision works too:

sage: f(s.O(2),t)
s^2 + s*t + O(s, t)^3
sage: f(f,f)
2*s^4 + 4*s^3*t + 2*s^2*t^2 + 4*s^5 + 8*s^4*t + 4*s^3*t^2 + 16*s^6 +
34*s^5*t + 20*s^4*t^2 + 2*s^3*t^3 + O(s, t)^7
sage: t(f,f)
s^2 + s*t + s^3 + s^2*t + 3*s^4 + 3*s^3*t + O(s, t)^5
sage: t(0,f) == s(f,0)
True

The subs syntax works as expected:

sage: r0 = -t^2 - s*t^3 - 2*t^6 + s^7 + s^5*t^2 + R.O(10)
sage: r1 = s^4 - s*t^4 + s^6*t - 4*s^2*t^5 - 6*s^3*t^5 + R.O(10)
sage: r2 = 2*s^3*t^2 - 2*s*t^4 - 2*s^3*t^4 + s*t^7 + R.O(10)
sage: r0.subs({t:r2,s:r1})
-4*s^6*t^4 + 8*s^4*t^6 - 4*s^2*t^8 + 8*s^6*t^6 - 8*s^4*t^8 - 4*s^4*t^9
+ 4*s^2*t^11 - 4*s^6*t^8 + O(s, t)^15
sage: r0.subs({t:r2,s:r1}) == r0(r1,r2)
True

Construct ring homomorphisms from one power series ring to another:

sage: A.<a,b> = PowerSeriesRing(QQ)
sage: X.<x,y> = PowerSeriesRing(QQ)

sage: phi = Hom(A,X)([x,2*y]); phi
Ring morphism:
  From: Multivariate Power Series Ring in a, b over Rational Field
  To:   Multivariate Power Series Ring in x, y over Rational Field
  Defn: a |--> x
        b |--> 2*y

sage: phi(a+b+3*a*b^2 + A.O(5))
x + 2*y + 12*x*y^2 + O(x, y)^5

Multiplicative inversion of power series:

sage: h = 1 + s + t + s*t + s^2*t^2 + 3*s^4 + 3*s^3*t + R.O(5)
sage: k = h^-1; k
1 - s - t + s^2 + s*t + t^2 - s^3 - s^2*t - s*t^2 - t^3 - 2*s^4 -
2*s^3*t + s*t^3 + t^4 + O(s, t)^5
sage: h*k
1 + O(s, t)^5

sage: f = 1 - 5*s^29 - 5*s^28*t + 4*s^18*t^35 + \
....: 4*s^17*t^36 - s^45*t^25 - s^44*t^26 + s^7*t^83 + \
....: s^6*t^84 + R.O(101)
sage: h = ~f; h
1 + 5*s^29 + 5*s^28*t - 4*s^18*t^35 - 4*s^17*t^36 + 25*s^58 + 50*s^57*t
+ 25*s^56*t^2 + s^45*t^25 + s^44*t^26 - 40*s^47*t^35 - 80*s^46*t^36
- 40*s^45*t^37 + 125*s^87 + 375*s^86*t + 375*s^85*t^2 + 125*s^84*t^3
- s^7*t^83 - s^6*t^84 + 10*s^74*t^25 + 20*s^73*t^26 + 10*s^72*t^27
+ O(s, t)^101
sage: h*f
1 + O(s, t)^101

AUTHORS:

• Niles Johnson (07/2010): initial code

• Simon King (08/2012): Use category and coercion framework, trac ticket #13412

class sage.rings.multi_power_series_ring_element.MO(x)

Bases: object

Object representing a zero element with given precision.

EXAMPLES:

sage: R.<u,v> = QQ[[]]
sage: m = O(u, v)
sage: m^4
0 + O(u, v)^4
sage: m^1
0 + O(u, v)^1

sage: T.<a,b,c> = PowerSeriesRing(ZZ,3)
sage: z = O(a, b, c)
sage: z^1
0 + O(a, b, c)^1
sage: 1 + a + z^1
1 + O(a, b, c)^1

sage: w = 1 + a + O(a, b, c)^2; w
1 + a + O(a, b, c)^2
sage: w^2
1 + 2*a + O(a, b, c)^2

class sage.rings.multi_power_series_ring_element.MPowerSeries(parent, x=0, prec=+ Infinity, is_gen=False, check=False)

Bases: sage.rings.power_series_ring_element.PowerSeries

Multivariate power series; these are the elements of Multivariate Power Series Rings.

INPUT:

• parent – A multivariate power series.

• x – The element (default: 0). This can be another MPowerSeries object, or an element of one of the following:

  • the background univariate power series ring

  • the foreground polynomial ring

  • a ring that coerces to one of the above two

• prec – (default: infinity) The precision

• is_gen – (default: False) Is this element one of the generators?

• check – (default: False) Needed by univariate power series class

EXAMPLES:

Construct multivariate power series from generators:

sage: S.<s,t> = PowerSeriesRing(ZZ)
sage: f = s + 4*t + 3*s*t
sage: f in S
True
sage: f = f.add_bigoh(4); f
s + 4*t + 3*s*t + O(s, t)^4
sage: g = 1 + s + t - s*t + S.O(5); g
1 + s + t - s*t + O(s, t)^5


sage: T = PowerSeriesRing(GF(3),5,'t'); T
Multivariate Power Series Ring in t0, t1, t2, t3, t4 over Finite
Field of size 3
sage: t = T.gens()
sage: w = t[0] - 2*t[1]*t[3] + 5*t[4]^3 - t[0]^3*t[2]^2; w
t0 + t1*t3 - t4^3 - t0^3*t2^2
sage: w = w.add_bigoh(5); w
t0 + t1*t3 - t4^3 + O(t0, t1, t2, t3, t4)^5
sage: w in T
True

sage: w = t[0] - 2*t[0]*t[2] + 5*t[4]^3 - t[0]^3*t[2]^2 + T.O(6)
sage: w
t0 + t0*t2 - t4^3 - t0^3*t2^2 + O(t0, t1, t2, t3, t4)^6

Get random elements:

sage: S.random_element(4) # random
-2*t + t^2 - 12*s^3 + O(s, t)^4

sage: T.random_element(10) # random
-t1^2*t3^2*t4^2 + t1^5*t3^3*t4 + O(t0, t1, t2, t3, t4)^10

Convert elements from polynomial rings:

sage: R = PolynomialRing(ZZ,5,T.variable_names())
sage: t = R.gens()
sage: r = -t[2]*t[3] + t[3]^2 + t[4]^2
sage: T(r)
-t2*t3 + t3^2 + t4^2
sage: r.parent()
Multivariate Polynomial Ring in t0, t1, t2, t3, t4 over Integer Ring
sage: r in T
True

O(prec)

Return a multivariate power series of precision prec obtained by truncating self at precision prec.

This is the same as add_bigoh().

EXAMPLES:

sage: B.<x,y> = PowerSeriesRing(QQ); B
Multivariate Power Series Ring in x, y over Rational Field
sage: r = 1 - x*y + x^2
sage: r.O(4)
1 + x^2 - x*y + O(x, y)^4
sage: r.O(2)
1 + O(x, y)^2

Note that this does not change self:

sage: r
1 + x^2 - x*y

V(n)

If

\[f = \sum a_{m_0, \ldots, m_k} x_0^{m_0} \cdots x_k^{m_k},\]

then this function returns

\[\sum a_{m_0, \ldots, m_k} x_0^{n m_0} \cdots x_k^{n m_k}.\]

The total-degree precision of the output is n times the precision of self.

EXAMPLES:

sage: H = QQ[['x,y,z']]
sage: (x,y,z) = H.gens()
sage: h = -x*y^4*z^7 - 1/4*y*z^12 + 1/2*x^7*y^5*z^2 \
+ 2/3*y^6*z^8 + H.O(15)
sage: h.V(3)
-x^3*y^12*z^21 - 1/4*y^3*z^36 + 1/2*x^21*y^15*z^6 + 2/3*y^18*z^24 + O(x, y, z)^45

add_bigoh(prec)

Return a multivariate power series of precision prec obtained by truncating self at precision prec.

This is the same as O().

EXAMPLES:

sage: B.<x,y> = PowerSeriesRing(QQ); B
Multivariate Power Series Ring in x, y over Rational Field
sage: r = 1 - x*y + x^2
sage: r.add_bigoh(4)
1 + x^2 - x*y + O(x, y)^4
sage: r.add_bigoh(2)
1 + O(x, y)^2

Note that this does not change self:

sage: r
1 + x^2 - x*y

coefficients()

Return a dict of monomials and coefficients.

EXAMPLES:

sage: R.<s,t> = PowerSeriesRing(ZZ); R
Multivariate Power Series Ring in s, t over Integer Ring
sage: f = 1 + t + s + s*t + R.O(3)
sage: f.coefficients()
{s*t: 1, t: 1, s: 1, 1: 1}
sage: (f^2).coefficients()
{t^2: 1, s*t: 4, s^2: 1, t: 2, s: 2, 1: 1}

sage: g = f^2 + f - 2; g
3*s + 3*t + s^2 + 5*s*t + t^2 + O(s, t)^3
sage: cd = g.coefficients()
sage: g2 = sum(k*v for (k,v) in cd.items()); g2
3*s + 3*t + s^2 + 5*s*t + t^2
sage: g2 == g.truncate()
True

constant_coefficient()

Return constant coefficient of self.

EXAMPLES:

sage: R.<a,b,c> = PowerSeriesRing(ZZ); R
Multivariate Power Series Ring in a, b, c over Integer Ring
sage: f = 3 + a + b - a*b - b*c - a*c + R.O(4)
sage: f.constant_coefficient()
3
sage: f.constant_coefficient().parent()
Integer Ring

degree()

Return degree of underlying polynomial of self.

EXAMPLES:

sage: B.<x,y> = PowerSeriesRing(QQ)
sage: B
Multivariate Power Series Ring in x, y over Rational Field
sage: r = 1 - x*y + x^2
sage: r = r.add_bigoh(4); r
1 + x^2 - x*y + O(x, y)^4
sage: r.degree()
2

derivative(*args)

The formal derivative of this power series, with respect to variables supplied in args.

EXAMPLES:

sage: T.<a,b> = PowerSeriesRing(ZZ,2)
sage: f = a + b + a^2*b + T.O(5)
sage: f.derivative(a)
1 + 2*a*b + O(a, b)^4
sage: f.derivative(a,2)
2*b + O(a, b)^3
sage: f.derivative(a,a)
2*b + O(a, b)^3
sage: f.derivative([a,a])
2*b + O(a, b)^3
sage: f.derivative(a,5)
0 + O(a, b)^0
sage: f.derivative(a,6)
0 + O(a, b)^0

dict()

Return underlying dictionary with keys the exponents and values the coefficients of this power series.

EXAMPLES:

sage: M = PowerSeriesRing(QQ,4,'t',sparse=True); M
Sparse Multivariate Power Series Ring in t0, t1, t2, t3 over
Rational Field

sage: M.inject_variables()
Defining t0, t1, t2, t3

sage: m = 2/3*t0*t1^15*t3^48 - t0^15*t1^21*t2^28*t3^5
sage: m2 = 1/2*t0^12*t1^29*t2^46*t3^6 - 1/4*t0^39*t1^5*t2^23*t3^30 + M.O(100)
sage: s = m + m2
sage: s.dict()
{(1, 15, 0, 48): 2/3,
 (12, 29, 46, 6): 1/2,
 (15, 21, 28, 5): -1,
 (39, 5, 23, 30): -1/4}

egf()

Method from univariate power series not yet implemented

exp(prec=+ Infinity)

Exponentiate the formal power series.

INPUT:

• prec – Integer or infinity. The degree to truncate the result to.

OUTPUT:

The exponentiated multivariate power series as a new multivariate power series.

EXAMPLES:

sage: T.<a,b> = PowerSeriesRing(ZZ,2)
sage: f = a + b + a*b + T.O(3)
sage: exp(f)
1 + a + b + 1/2*a^2 + 2*a*b + 1/2*b^2 + O(a, b)^3
sage: f.exp()
1 + a + b + 1/2*a^2 + 2*a*b + 1/2*b^2 + O(a, b)^3
sage: f.exp(prec=2)
1 + a + b + O(a, b)^2
sage: log(exp(f)) - f
0 + O(a, b)^3

If the power series has a constant coefficient \(c\) and \(\exp(c)\) is transcendental, then \(\exp(f)\) would have to be a power series over the SymbolicRing. These are not yet implemented and therefore such cases raise an error:

sage: g = 2+f
sage: exp(g)
Traceback (most recent call last):
...
TypeError: unsupported operand parent(s) for *: 'Symbolic Ring' and
'Power Series Ring in Tbg over Multivariate Polynomial Ring in a, b
over Rational Field'

Another workaround for this limitation is to change base ring to one which is closed under exponentiation, such as \(\RR\) or \(\CC\):

sage: exp(g.change_ring(RDF))
7.38905609... + 7.38905609...*a + 7.38905609...*b + 3.69452804...*a^2 +
14.7781121...*a*b + 3.69452804...*b^2 + O(a, b)^3

If no precision is specified, the default precision is used:

sage: T.default_prec()
12
sage: exp(a)
1 + a + 1/2*a^2 + 1/6*a^3 + 1/24*a^4 + 1/120*a^5 + 1/720*a^6 + 1/5040*a^7 +
1/40320*a^8 + 1/362880*a^9 + 1/3628800*a^10 + 1/39916800*a^11 + O(a, b)^12
sage: a.exp(prec=5)
1 + a + 1/2*a^2 + 1/6*a^3 + 1/24*a^4 + O(a, b)^5
sage: exp(a + T.O(5))
1 + a + 1/2*a^2 + 1/6*a^3 + 1/24*a^4 + O(a, b)^5

exponents()

Return a list of tuples which hold the exponents of each monomial of self.

EXAMPLES:

sage: H = QQ[['x,y']]
sage: (x,y) = H.gens()
sage: h = -y^2 - x*y^3 - 6/5*y^6 - x^7 + 2*x^5*y^2 + H.O(10)
sage: h
-y^2 - x*y^3 - 6/5*y^6 - x^7 + 2*x^5*y^2 + O(x, y)^10
sage: h.exponents()
[(0, 2), (1, 3), (0, 6), (7, 0), (5, 2)]

integral(*args)

The formal integral of this multivariate power series, with respect to variables supplied in args.

The variable sequence args can contain both variables and counts; for the syntax, see derivative_parse().

EXAMPLES:

sage: T.<a,b> = PowerSeriesRing(QQ,2)
sage: f = a + b + a^2*b + T.O(5)
sage: f.integral(a, 2)
1/6*a^3 + 1/2*a^2*b + 1/12*a^4*b + O(a, b)^7
sage: f.integral(a, b)
1/2*a^2*b + 1/2*a*b^2 + 1/6*a^3*b^2 + O(a, b)^7
sage: f.integral(a, 5)
1/720*a^6 + 1/120*a^5*b + 1/2520*a^7*b + O(a, b)^10

Only integration with respect to variables works:

sage: f.integral(a+b)
Traceback (most recent call last):
...
ValueError: a + b is not a variable

Warning

Coefficient division.

If the base ring is not a field (e.g. \(ZZ\)), or if it has a non-zero characteristic, (e.g. \(ZZ/3ZZ\)), integration is not always possible while staying with the same base ring. In the first case, Sage will report that it has not been able to coerce some coefficient to the base ring:

sage: T.<a,b> = PowerSeriesRing(ZZ,2)
sage: f = a + T.O(5)
sage: f.integral(a)
Traceback (most recent call last):
...
TypeError: no conversion of this rational to integer

One can get the correct result by changing the base ring first:

sage: f.change_ring(QQ).integral(a)
1/2*a^2 + O(a, b)^6

However, a correct result is returned even without base change if the denominator cancels:

sage: f = 2*b + T.O(5)
sage: f.integral(b)
b^2 + O(a, b)^6

In non-zero characteristic, Sage will report that a zero division occurred

sage: T.<a,b> = PowerSeriesRing(Zmod(3),2)
sage: (a^3).integral(a)
a^4
sage: (a^2).integral(a)
Traceback (most recent call last):
...
ZeroDivisionError: inverse of Mod(0, 3) does not exist

is_nilpotent()

Return True if self is nilpotent. This occurs if

• self has finite precision and positive valuation, or

• self is constant and nilpotent in base ring.

Otherwise, return False.

Warning

This is so far just a sufficient condition, so don’t trust a False output to be legit!

Todo

What should we do about this method? Is nilpotency of a power series even decidable (assuming a nilpotency oracle in the base ring)? And I am not sure that returning True just because the series has finite precision and zero constant term is a good idea.

EXAMPLES:

sage: R.<a,b,c> = PowerSeriesRing(Zmod(8)); R
Multivariate Power Series Ring in a, b, c over Ring of integers
modulo 8
sage: f = a + b + c + a^2*c
sage: f.is_nilpotent()
False
sage: f = f.O(4); f
a + b + c + a^2*c + O(a, b, c)^4
sage: f.is_nilpotent()
True

sage: g = R(2)
sage: g.is_nilpotent()
True
sage: (g.O(4)).is_nilpotent()
True

sage: S = R.change_ring(QQ)
sage: S(g).is_nilpotent()
False
sage: S(g.O(4)).is_nilpotent()
False

is_square()

Method from univariate power series not yet implemented.

is_unit()

A multivariate power series is a unit if and only if its constant coefficient is a unit.

EXAMPLES:

sage: R.<a,b> = PowerSeriesRing(ZZ); R
Multivariate Power Series Ring in a, b over Integer Ring
sage: f = 2 + a^2 + a*b + a^3 + R.O(9)
sage: f.is_unit()
False
sage: f.base_extend(QQ).is_unit()
True
sage: (O(a,b)^0).is_unit()
False

laurent_series()

Not implemented for multivariate power series.

list()

Doesn’t make sense for multivariate power series. Multivariate polynomials don’t have list of coefficients either.

log(prec=+ Infinity)

Return the logarithm of the formal power series.

INPUT:

• prec – Integer or infinity. The degree to truncate the result to.

OUTPUT:

The logarithm of the multivariate power series as a new multivariate power series.

EXAMPLES:

sage: T.<a,b> = PowerSeriesRing(ZZ,2)
sage: f = 1 + a + b + a*b + T.O(5)
sage: f.log()
a + b - 1/2*a^2 - 1/2*b^2 + 1/3*a^3 + 1/3*b^3 - 1/4*a^4 - 1/4*b^4 + O(a, b)^5
sage: log(f)
a + b - 1/2*a^2 - 1/2*b^2 + 1/3*a^3 + 1/3*b^3 - 1/4*a^4 - 1/4*b^4 + O(a, b)^5
sage: exp(log(f)) - f
0 + O(a, b)^5

If the power series has a constant coefficient \(c\) and \(\exp(c)\) is transcendental, then \(\exp(f)\) would have to be a power series over the SymbolicRing. These are not yet implemented and therefore such cases raise an error:

sage: g = 2+f
sage: log(g)
Traceback (most recent call last):
...
TypeError: unsupported operand parent(s) for -: 'Symbolic Ring' and 'Power
Series Ring in Tbg over Multivariate Polynomial Ring in a, b over Rational Field'

Another workaround for this limitation is to change base ring to one which is closed under exponentiation, such as \(\RR\) or \(\CC\):

sage: log(g.change_ring(RDF))
1.09861228... + 0.333333333...*a + 0.333333333...*b - 0.0555555555...*a^2
+ 0.222222222...*a*b - 0.0555555555...*b^2 + 0.0123456790...*a^3
- 0.0740740740...*a^2*b - 0.0740740740...*a*b^2 + 0.0123456790...*b^3
- 0.00308641975...*a^4 + 0.0246913580...*a^3*b + 0.0246913580...*a*b^3
- 0.00308641975...*b^4 + O(a, b)^5

monomials()

Return a list of monomials of self.

These are the keys of the dict returned by coefficients().

EXAMPLES:

sage: R.<a,b,c> = PowerSeriesRing(ZZ); R
Multivariate Power Series Ring in a, b, c over Integer Ring
sage: f = 1 + a + b - a*b - b*c - a*c + R.O(4)
sage: sorted(f.monomials())
[b*c, a*c, a*b, b, a, 1]
sage: f = 1 + 2*a + 7*b - 2*a*b - 4*b*c - 13*a*c + R.O(4)
sage: sorted(f.monomials())
[b*c, a*c, a*b, b, a, 1]
sage: f = R.zero()
sage: f.monomials()
[]

ogf()

Method from univariate power series not yet implemented

padded_list()

Method from univariate power series not yet implemented.

polynomial()

Return the underlying polynomial of self as an element of the underlying multivariate polynomial ring (the “foreground polynomial ring”).

EXAMPLES:

sage: M = PowerSeriesRing(QQ,4,'t'); M
Multivariate Power Series Ring in t0, t1, t2, t3 over Rational
Field
sage: t = M.gens()
sage: f = 1/2*t[0]^3*t[1]^3*t[2]^2 + 2/3*t[0]*t[2]^6*t[3]             - t[0]^3*t[1]^3*t[3]^3 - 1/4*t[0]*t[1]*t[2]^7 + M.O(10)
sage: f
1/2*t0^3*t1^3*t2^2 + 2/3*t0*t2^6*t3 - t0^3*t1^3*t3^3
- 1/4*t0*t1*t2^7 + O(t0, t1, t2, t3)^10

sage: f.polynomial()
1/2*t0^3*t1^3*t2^2 + 2/3*t0*t2^6*t3 - t0^3*t1^3*t3^3
- 1/4*t0*t1*t2^7

sage: f.polynomial().parent()
Multivariate Polynomial Ring in t0, t1, t2, t3 over Rational Field

Contrast with truncate():

sage: f.truncate()
1/2*t0^3*t1^3*t2^2 + 2/3*t0*t2^6*t3 - t0^3*t1^3*t3^3 - 1/4*t0*t1*t2^7
sage: f.truncate().parent()
Multivariate Power Series Ring in t0, t1, t2, t3 over Rational Field

prec()

Return precision of self.

EXAMPLES:

sage: R.<a,b,c> = PowerSeriesRing(ZZ); R
Multivariate Power Series Ring in a, b, c over Integer Ring
sage: f = 3 + a + b - a*b - b*c - a*c + R.O(4)
sage: f.prec()
4
sage: f.truncate().prec()
+Infinity

quo_rem(other, precision=None)

Return the pair of quotient and remainder for the increasing power division of self by other.

If \(a\) and \(b\) are two elements of a power series ring \(R[[x_1, x_2, \cdots, x_n]]\) such that the trailing term of \(b\) is invertible in \(R\), then the pair of quotient and remainder for the increasing power division of \(a\) by \(b\) is the unique pair \((u, v) \in R[[x_1, x_2, \cdots, x_n]] \times R[x_1, x_2, \cdots, x_n]\) such that \(a = bu + v\) and such that no monomial appearing in \(v\) divides the trailing monomial (trailing_monomial()) of \(b\). Note that this depends on the order of the variables.

This method returns both quotient and remainder as power series, even though in mathematics, the remainder for the increasing power division of two power series is a polynomial. This is because Sage’s power series come with a precision, and that precision is not always sufficient to determine the remainder completely. Disregarding this issue, the polynomial() method can be used to recast the remainder as an actual polynomial.

INPUT:

• other – an element of the same power series ring as self such that the trailing term of other is invertible in self (this is automatically satisfied if the base ring is a field, unless other is zero)

• precision – (default: the default precision of the parent of self) nonnegative integer, determining the precision to be cast on the resulting quotient and remainder if both self and other have infinite precision (ignored otherwise); note that the resulting precision might be lower than this integer

EXAMPLES:

sage: R.<a,b,c> = PowerSeriesRing(ZZ)
sage: f = 1 + a + b - a*b + R.O(3)
sage: g = 1 + 2*a - 3*a*b + R.O(3)
sage: q, r = f.quo_rem(g); q, r
(1 - a + b + 2*a^2 + O(a, b, c)^3, 0 + O(a, b, c)^3)
sage: f == q*g+r
True

sage: q, r = (a*f).quo_rem(g); q, r
(a - a^2 + a*b + 2*a^3 + O(a, b, c)^4, 0 + O(a, b, c)^4)
sage: a*f == q*g+r
True

sage: q, r = (a*f).quo_rem(a*g); q, r
(1 - a + b + 2*a^2 + O(a, b, c)^3, 0 + O(a, b, c)^4)
sage: a*f == q*(a*g)+r
True

sage: q, r = (a*f).quo_rem(b*g); q, r
(a - 3*a^2 + O(a, b, c)^3, a + a^2 + O(a, b, c)^4)
sage: a*f == q*(b*g)+r
True

Trying to divide two polynomials, we run into the issue that there is no natural setting for the precision of the quotient and remainder (and if we wouldn’t set a precision, the algorithm would never terminate). Here, default precision comes to our help:

sage: (1+a^3).quo_rem(a+a^2)
(a^2 - a^3 + a^4 - a^5 + a^6 - a^7 + a^8 - a^9 + a^10 + O(a, b, c)^11, 1 + O(a, b, c)^12)

sage: (1+a^3+a*b).quo_rem(b+c)
(a + O(a, b, c)^11, 1 - a*c + a^3 + O(a, b, c)^12)
sage: (1+a^3+a*b).quo_rem(b+c, precision=17)
(a + O(a, b, c)^16, 1 - a*c + a^3 + O(a, b, c)^17)

sage: (a^2+b^2+c^2).quo_rem(a+b+c)
(a - b - c + O(a, b, c)^11, 2*b^2 + 2*b*c + 2*c^2 + O(a, b, c)^12)

sage: (a^2+b^2+c^2).quo_rem(1/(1+a+b+c))
(a^2 + b^2 + c^2 + a^3 + a^2*b + a^2*c + a*b^2 + a*c^2 + b^3 + b^2*c + b*c^2 + c^3 + O(a, b, c)^14,
 0)

sage: (a^2+b^2+c^2).quo_rem(a/(1+a+b+c))
(a + a^2 + a*b + a*c + O(a, b, c)^13, b^2 + c^2)

sage: (1+a+a^15).quo_rem(a^2)
(0 + O(a, b, c)^10, 1 + a + O(a, b, c)^12)
sage: (1+a+a^15).quo_rem(a^2, precision=15)
(0 + O(a, b, c)^13, 1 + a + O(a, b, c)^15)
sage: (1+a+a^15).quo_rem(a^2, precision=16)
(a^13 + O(a, b, c)^14, 1 + a + O(a, b, c)^16)

Illustrating the dependency on the ordering of variables:

sage: (1+a+b).quo_rem(b+c)
(1 + O(a, b, c)^11, 1 + a - c + O(a, b, c)^12)
sage: (1+b+c).quo_rem(c+a)
(0 + O(a, b, c)^11, 1 + b + c + O(a, b, c)^12)
sage: (1+c+a).quo_rem(a+b)
(1 + O(a, b, c)^11, 1 - b + c + O(a, b, c)^12)

shift(n)

Doesn’t make sense for multivariate power series.

solve_linear_de(prec=+ Infinity, b=None, f0=None)

Not implemented for multivariate power series.

sqrt()

Method from univariate power series not yet implemented. Depends on square root method for multivariate polynomials.

square_root()

Method from univariate power series not yet implemented. Depends on square root method for multivariate polynomials.

trailing_monomial()

Return the trailing monomial of self.

This is defined here as the lowest term of the underlying polynomial.

EXAMPLES:

sage: R.<a,b,c> = PowerSeriesRing(ZZ)
sage: f = 1 + a + b - a*b + R.O(3)
sage: f.trailing_monomial()
1
sage: f = a^2*b^3*f; f
a^2*b^3 + a^3*b^3 + a^2*b^4 - a^3*b^4 + O(a, b, c)^8
sage: f.trailing_monomial()
a^2*b^3

truncate(prec=+ Infinity)

Return infinite precision multivariate power series formed by truncating self at precision prec.

EXAMPLES:

sage: M = PowerSeriesRing(QQ,4,'t'); M
Multivariate Power Series Ring in t0, t1, t2, t3 over Rational Field
sage: t = M.gens()
sage: f = 1/2*t[0]^3*t[1]^3*t[2]^2 + 2/3*t[0]*t[2]^6*t[3]             - t[0]^3*t[1]^3*t[3]^3 - 1/4*t[0]*t[1]*t[2]^7 + M.O(10)
sage: f
1/2*t0^3*t1^3*t2^2 + 2/3*t0*t2^6*t3 - t0^3*t1^3*t3^3
- 1/4*t0*t1*t2^7 + O(t0, t1, t2, t3)^10

sage: f.truncate()
1/2*t0^3*t1^3*t2^2 + 2/3*t0*t2^6*t3 - t0^3*t1^3*t3^3
- 1/4*t0*t1*t2^7
sage: f.truncate().parent()
Multivariate Power Series Ring in t0, t1, t2, t3 over Rational Field

Contrast with polynomial:

sage: f.polynomial()
1/2*t0^3*t1^3*t2^2 + 2/3*t0*t2^6*t3 - t0^3*t1^3*t3^3 - 1/4*t0*t1*t2^7
sage: f.polynomial().parent()
Multivariate Polynomial Ring in t0, t1, t2, t3 over Rational Field

valuation()

Return the valuation of self.

The valuation of a power series \(f\) is the highest nonnegative integer \(k\) less or equal to the precision of \(f\) and such that the coefficient of \(f\) before each term of degree \(< k\) is zero. (If such an integer does not exist, then the valuation is the precision of \(f\) itself.)

EXAMPLES:

sage: R.<a,b> = PowerSeriesRing(GF(4949717)); R
Multivariate Power Series Ring in a, b over Finite Field of
size 4949717
sage: f = a^2 + a*b + a^3 + R.O(9)
sage: f.valuation()
2
sage: g = 1 + a + a^3
sage: g.valuation()
0
sage: R.zero().valuation()
+Infinity

valuation_zero_part()

Doesn’t make sense for multivariate power series; valuation zero with respect to which variable?

variable()

Doesn’t make sense for multivariate power series.

variables()

Return tuple of variables occurring in self.

EXAMPLES:

sage: T = PowerSeriesRing(GF(3),5,'t'); T
Multivariate Power Series Ring in t0, t1, t2, t3, t4 over
Finite Field of size 3
sage: t = T.gens()
sage: w = t[0] - 2*t[0]*t[2] + 5*t[4]^3 - t[0]^3*t[2]^2 + T.O(6)
sage: w
t0 + t0*t2 - t4^3 - t0^3*t2^2 + O(t0, t1, t2, t3, t4)^6
sage: w.variables()
(t0, t2, t4)

sage.rings.multi_power_series_ring_element.is_MPowerSeries(f)

Return True if f is a multivariate power series.