# Algorithms for the arithmetic-geometric mean¶

With complex variables, it is convenient to work with the univariate function $$M(z) = \operatorname{agm}(1,z)$$. The general case is given by $$\operatorname{agm}(a,b) = a M(1,b/a)$$.

## Functional equation¶

If the real part of z initially is not completely nonnegative, we apply the functional equation $$M(z) = (z+1) M(u) / 2$$ where $$u = \sqrt{z} / (z+1)$$.

Note that u has nonnegative real part, absent rounding error. It is not a problem for correctness if rounding makes the interval contain negative points, as this just inflates the final result.

For the derivative, the functional equation becomes $$M'(z) = [M(u) - (z-1) M'(u) / ((1+z) \sqrt{z})] / 2$$.

## AGM iteration¶

Once z is in the right half plane, we can apply the AGM iteration ($$2a_{n+1} = a_n + b_n, b_{n+1}^2 = a_n b_n$$) directly. The correct square root is given by $$\sqrt{a} \sqrt{b}$$, which is computed as $$\sqrt{ab}, i \sqrt{-ab}, -i \sqrt{-ab}, \sqrt{a} \sqrt{b}$$ respectively if both a and b have positive real part, nonnegative imaginary part, nonpositive imaginary part, or otherwise.

The iteration should be terminated when $$a_n$$ and $$b_n$$ are close enough. For positive real variables, we can simply take lower and upper bounds to get a correct enclosure at this point. For complex variables, it is shown in [Dup2006], p. 87 that, for z with nonnegative real part, $$|M(z) - a_n| \le |a_n - b_n|$$, giving a convenient error bound.

Rather than running the AGM iteration until $$a_n$$ and $$b_n$$ agree to $$p$$ bits, it is slightly more efficient to iterate until they agree to about $$p/10$$ bits and finish with a series expansion. With $$z = (a-b)/(a+b)$$, we have

$\operatorname{agm}(a,b) = \frac{(a+b) \pi}{4 K(z^2)},$

valid at least when $$|z| < 1$$ and $$a, b$$ have nonnegative real part, and

$\frac{\pi}{4 K(z^2)} = \tfrac{1}{2} - \tfrac{1}{8} z^2 - \tfrac{5}{128} z^4 - \tfrac{11}{512} z^6 - \tfrac{469}{32768} z^8 + \ldots$

where the tail is bounded by $$\sum_{k=10}^{\infty} |z|^k/64$$.

## First derivative¶

Assuming that z is exact and that $$|\arg(z)| \le 3 \pi / 4$$, we compute $$(M(z), M'(z))$$ simultaneously using a finite difference.

The basic inequality we need is $$|M(z)| \le \max(1, |z|)$$, which is an immediate consequence of the AGM iteration.

By Cauchy’s integral formula, $$|M^{(k)}(z) / k!| \le C D^k$$ where $$C = \max(1, |z| + r)$$ and $$D = 1/r$$, for any $$0 < r < |z|$$ (we choose r to be of the order $$|z| / 4$$). Taylor expansion now gives

\begin{align}\begin{aligned}\left|\frac{M(z+h) - M(z)}{h} - M'(z)\right| \le \frac{C D^2 h}{1 - D h}\\\left|\frac{M(z+h) - M(z-h)}{2h} - M'(z)\right| \le \frac{C D^3 h^2}{1 - D h}\\\left|\frac{M(z+h) + M(z-h)}{2} - M(z)\right| \le \frac{C D^2 h^2}{1 - D h}\end{aligned}\end{align}

assuming that h is chosen so that it satisfies $$h D < 1$$.

The forward finite difference would require two function evaluations at doubled precision. We use the central difference as it only requires 1.5 times the precision.

When z is not exact, we evaluate at the midpoint as above and bound the propagated error using derivatives. Again by Cauchy’s integral formula, we have

\begin{align}\begin{aligned}|M'(z+\varepsilon)| \le \frac{\max(1, |z|+|\varepsilon|+r)}{r}\\|M''(z+\varepsilon)| \le \frac{2 \max(1, |z|+|\varepsilon|+r)}{r^2}\end{aligned}\end{align}

assuming that the circle centered on z with radius $$|\varepsilon| + r$$ does not cross the negative half axis. We choose r of order $$|z| / 2$$ and verify that all assumptions hold.

## Higher derivatives¶

The function $$W(z) = 1 / M(z)$$ is D-finite. The coefficients of $$W(z+x) = \sum_{k=0}^{\infty} c_k x^k$$ satisfy

$-2 z (z^2-1) c_2 = (3z^2-1) c_1 + z c_0,$
$-(k+2)(k+3) z (z^2-1) c_{k+3} = (k+2)^2 (3z^2-1) c_{k+2} + (3k(k+3)+7)z c_{k+1} + (k+1)^2 c_{k}$

in general, and

$-(k+2)^2 c_{k+2} = (3k(k+3)+7) c_{k+1} + (k+1)^2 c_{k}$

when $$z = 1$$.