Error terms for Gauss-Legendre quadrature
-----------------------------------------

order                  error term
n = 10          1.2e-24     * f^(2n) (x)
n = 21          5.0e-64     * f^(2n) (x)
n = 43          1.7e-156    * f^(2n) (x)
n = 87          2.0e-368    * f^(2n) (x)
n = 175         1.1e-845    * f^(2n) (x)
