Processing math: 100%

$\newcommand\al{\alpha} \newcommand\be{\beta} \newcommand\de{\delta} \newcommand\De{\Delta} \newcommand\eps{\epsilon} \newcommand\ga{\gamma} \newcommand\Ga{\Gamma} \newcommand\ka{\kappa} \newcommand\la{\lambda} \newcommand\La{\Lambda} \newcommand\om{\omega} \newcommand\Om{\Omega} \newcommand\sg{\sigma} \newcommand\Sg{\Sigma} \renewcommand\th{\theta} %--- Latex uses \th for a Norse character \newcommand\Th{\Theta} \newcommand\vphi{\varphi} % % some calligraphy % \newcommand\cA{{\mathcal A}} \newcommand\cB{{\mathcal B}} \newcommand\cC{{\mathcal C}} \newcommand\cD{{\mathcal D}} \newcommand\cE{{\mathcal E}} \newcommand\cF{{\mathcal F}} \newcommand\cG{{\mathcal G}} \newcommand\cH{{\mathcal H}} \newcommand\cI{{\mathcal I}} \newcommand\cJ{{\mathcal J}} \newcommand\cK{{\mathcal K}} \newcommand\cL{{\mathcal L}} \newcommand\cM{{\mathcal M}} \newcommand\cN{{\mathcal N}} \newcommand\cO{{\mathcal O}} \newcommand\cP{{\mathcal P}} \newcommand\cQ{{\mathcal Q}} \newcommand\cR{{\mathcal R}} \newcommand\cS{{\mathcal S}} \newcommand\cT{{\mathcal T}} \newcommand\cU{{\mathcal U}} \newcommand\cV{{\mathcal V}} % % fields and rings (and a semigroup) % \newcommand\cx{{\mathbb C}}% complexes \newcommand\fld{{\mathbb F}} \newcommand\flde{{\mathbb E}} \newcommand\ints{{\mathbb Z}} \newcommand\nn{{\mathbb N}}%non-negative integers \newcommand\re{{\mathbb R}}%reals \newcommand\rats{{\mathbb Q}} % % the really useful stuff % \newcommand\comp[1]{{\mkern2mu\overline{\mkern-2mu#1}}} \newcommand\diff{\mathbin{\mkern-1.5mu\setminus\mkern-1.5mu}}% for \setminus \newcommand\res{\mathbin{\mkern-2.0mu\restriction\mkern-2.0mu}} \newcommand\sbs{\subseteq} \newcommand\sps{\supseteq} \newcommand\seq[3]{#1_{#2},\ldots,#1_{#3}} \DeclareMathOperator{\supp}{supp} \DeclareMathOperator{\im}{im} \DeclareMathOperator{\row}{row} \newcommand\pmat[1]{\begin{pmatrix} #1 \end{pmatrix}} \newcommand\cprod{\mathbin{\square}} \newcommand\gbin[2]{\genfrac{[}{]}{0pt}{}{#1}{#2}} % % matrix theory % \newcommand\ip[2]{\langle#1,#2\rangle} \newcommand\one{{\bf1}} \DeclareMathOperator{\rk}{rk} \DeclareMathOperator{\tr}{tr} \DeclareMathOperator{\col}{col} \newcommand\mat[3]{\mathrm{Mat}_{#1\times #2}(#3)} \newcommand\sm[3]{\sum_{#1=#2}^{#3}} % % some group theory % \newcommand\aut[1]{{\rm Aut}(#1)} \newcommand\fx[1]{{\rm fix}(#1)}% ch2 \newcommand\grp[1]{\langle #1\rangle} \newcommand\nrml{\vartriangleleft} \newcommand\nrmleq{\trianglelefteq} \DeclareMathOperator{\Sym}{Sym} \newcommand\sym[1]{\Sym(#1)} \DeclareMathOperator{\Alt}{Alt} \newcommand\alt[1]{\Alt(#1)}$

Section8.2Fractions and Series

Many generating series can be expressed as rational functions, and such expressions provide useful information. We illustrate this. First we create a ring of polynomials, and the corresponding field of fractions. (We will not need the latter, as it happens.)

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Qu.<u> = QQ[]
F = FractionField(Qu)
Qu

Now we can work with our series $L(t)$ from the previous section.

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rf = (1-2*u+ u^3/(1+u+u^2))^(-1)
rf.numerator(), rf.denominator()

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rf.subs(u=1/2)

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Rt.<t> = QQ[[]]
rf.subs(u=t).O(9)

Note that t is the generator of Rt, which is $\rats[[t]]$ . So we have used subs() to convert a rational function to a power series.

We can also compute partial fraction decompositions. For these to be useful the zeros of the denominator should belong to the field we are working over.

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Cs = FractionField( PolynomialRing(CDF, 's'))
whole, parts = rf.subs(u=Cs.gen()).partial_fraction_decomposition() 
for p in parts:
    print p

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