On 1/29/07, Keith Goodman <kwgoodman@gmail.com> wrote:
On 1/29/07, Keith Goodman <kwgoodman@gmail.com> wrote:
On 1/29/07, Keith Goodman <kwgoodman@gmail.com> wrote:
On 1/29/07, Charles R Harris <charlesr.harris@gmail.com> wrote:
That's odd, the LSB bit of the double precision mantissa is only about 2.2e-16, so you can't *get* differences as small as 8.4e-22 without about 70 bit mantissa's. Hmmm, and extended double precision only has 63 bit mantissa's. Are you sure you are computing the error correctly?
That is odd.
8.4e-22 is just the output of the test script: abs(z - z0).max(). That abs is from python.
By playing around with x and y I can get all sorts of values for abs(z - z0).max(). I can get down to the e-23 range and to 2.2e-16. I've also seen e-18 and e-22.
Here is a setting for x and y that gives me a difference (using the unit test in this thread) of 4.54747e-13! That is huge---and a serious problem. I am sure I can get bigger.
# x data x = M.zeros((3,3)) x[0,0] = 9.0030140479499 x[0,1] = 9.0026474226671 x[0,2] = -9.0011270502873 x[1,0] = 9.0228605377994 x[1,1] = 9.0033715311274 x[1,2] = -9.0082367491299 x[2,0] = 9.0044783987583 x[2,1] = 0.0027488028057 x[2,2] = -9.0036113393360
# y data y = M.zeros((3,1)) y[0,0] =10.00088539878978 y[1,0] = 0.00667193234012 y[2,0] = 0.00032472712345
OK. I guess I should be looking at the fractional difference instead of the absolute difference. The fractional difference is of order e-16.