Poorly conditioned, easily evaluated sum for unit testing

Question

I am looking for examples of poorly conditioned sums which can rapidly be evaluated, for the purposes of unit testing.

I'm currently using the series representation for $\ln(2)$:

$$ \sum_{n=1}^{\infty} \frac{(-1)^{n+1}}{n} $$ which has an infinite summation condition number, but in double precision, I can only get the condition number to ~22 after an obscene number of terms (terrible for a unit test), and the error in the naive summation is not large. So I guess that this sum is numerically well-conditioned!

Answer 1

The condition number of sum $s(x) = \sum_{j=1}^n x_j$ is given by $$ \kappa(x) = \frac{\sum_{j=1}^n |x_j|}{|\sum_{j=1}^n x_j|} = \frac{s(|x|)}{|s(x)|}$$ and reflects the sums sensitivity to small changes in the input. Specifically, we have $$ \underset{\epsilon \rightarrow 0_+}{\lim}\sup \left\{ \frac{1}{\epsilon} \left|\frac{s(x+\Delta x) - s(x)}{s(x)} \right| \: : \: |\Delta x_j| \leq \epsilon |x_j|\right\} = \kappa(x).$$ If all $x_j$ have the same sign, then the sum is well-conditioned. If the $s(x) \approx 0$, but $s(|x|)$ is large, then the sum is ill-conditioned. A good example of a sum whose conditioning is well understood is the polynomial $p_n$ given by $$ p_n(x) = \sum_{j=0}^n \frac{x^j}{j!}$$ This is the Taylor polynomial of order $n$ for the natural exponential function at the point $x_0 = 0$. The sum is ill-conditioned for large negative values of $x$ and well-conditioned for all positive values of $x$. If $x$ is positive and $n$ is so large that $p_n(x) \approx e^x$ is a good approximation, then $1/e^{x} \approx p_n(-x)$ is a good approximation as well. In short, you have an ill-conditioned sum for which a good approximation is easy to obtain.