# 4.   Complex Analysis, Rational and Meromorphic Asymptotics

## Analytic transfer theorem for rational functions (common case)

If $h(z)$ is a rational function that is analytic at 0 and has a pole $\alpha=1/\beta$ of smallest modulus with multiplicity $M$ which is larger than the multiplicity of all other poles of smallest modulus, then $$[z^n]h(z) \sim c\beta^nn^{M-1}\quad\hbox{where}\quad c = {\alpha^M\over(M-1)!}\lim_{z\to\alpha} (z-\alpha)^Mh(z).$$

See Theorem IV.9 and Note IV.26 on page 256.

Example.The number of ways to make change for $n$ cents with pennies, nickels, dimes and quarters is $$[z^n]{1\over(1-z)(1-z^5)(1-z^{10})(1-z^{25})} ~\sim {n^3\over 3!}{1\over 1\cdot 5 \cdot 10 \cdot 25}={n^3\over 7500}$$ because $z=1$ is a pole of multiplicity 4 and $$\lim_{z\to 1}{1-z\over 1-z^t} = \lim_{z\to 1}{1\over 1 + z + z^2 + \ldots + z^{t-1}} = {1\over t}.$$

See Proposition IV.2 on page 258 and Slide 46 in Lecture 5.

With $h(z) = f(z)/g(z)$, another way to calculate the constant is to use l'Hôpital's rule, with the result $$c = M{(-\beta)^Mf(\alpha)\over g^{(M)}(\alpha)}.$$ See slide 30 in Lecture 4 or Theorem 4.1 in Analysis of Algorithms.

## Note IV.28

Supernecklaces. A "supernecklace" of the 3rd type is a labelled cycle of cycles (see page 125). Draw all the supernecklaces of the 3rd type of size $n$ for $n$ = 1, 2, 3, and 4. Then develop an asymptotic estimate of the number of supernecklaces of size $n$ by showing that $$[z^n]\ln\Bigl({1\over 1 - \ln{1\over 1-z}}\Bigr)\sim{1\over n}(1-e^{-1})^{-n}.$$ Hint: Take derivatives.

## Program IV.1

Compute the percentage of permutations having no singelton or doubleton cycles and compare with the asymptotic estimate from analytic combinatorics, for N = 10 and N = 20 .

## Program IV.2

Plot the derivative of the supernecklace GF (see Note IV.28) in the style of the plots in Lecture 4. Click here for access to the Java code in the lecture.

## IV.1

(D. Luo) Approximately how many ways are there to change $n$ cents in Canada (no pennies)?