| forum.alglib.net http://forum.alglib.net/ |
|
| Unexpected result for exponentialintegralen function http://forum.alglib.net/viewtopic.php?f=2&t=4610 |
Page 1 of 1 |
| Author: | Fred5962 [ Fri Aug 02, 2024 10:58 am ] |
| Post subject: | Unexpected result for exponentialintegralen function |
The function exponentialintegralen(x) replies -1.0 when x>170.0, while the function should decrease asymptotically to zero. There is a threshold effect at x=170.0 (replies 8,64881582108917E-77). Thank you for any clarification. |
|
| Author: | Fred5962 [ Tue Aug 13, 2024 8:17 am ] | ||
| Post subject: | Re: Unexpected result for exponentialintegralen function | ||
Typical polynomial and rational approximations show the obvious asymptotical trend of the exponential integral towards zero for large values of x (references in the attached graphic file). Why does the result of AlgLib switch to E1(x) = -1 when x>170?!
|
|||
| Author: | Fred5962 [ Tue Aug 20, 2024 12:48 pm ] |
| Post subject: | Re: Unexpected result for exponentialintegralen function |
The answer is found in specialfunctions.cpp file, at line 6749: Code: if( ((n<0||ae_fp_less(x,(double)(0)))||[b]ae_fp_greater(x,(double)(170)))[/b]||(ae_fp_eq(x,(double)(0))&&n<2) ) { result = (double)(-1); return result; } When the function exponentialintegralen is used with invalid parameters (x<0 or n<0), it returns (-1) to show that input parameters are out of the validity domain. Correct. But the middle expression: Code: ae_fp_greater(x,(double)(170)) in the test must not be here.The correct code can be found in the CEPHES script (where MAXLOG would be 170): Code: /* Cephes Math Library Release 2.8: June, 2000 Copyright 1985, 2000 by Stephen L. Moshier */ if( x > MAXLOG ) return( 0.0 ); Why has this original CEPHES code been changed? |
|
| Author: | Fred5962 [ Tue Aug 20, 2024 3:14 pm ] |
| Post subject: | Re: Unexpected result for exponentialintegralen function |
When the original script of CEPHES is kept - i.e. without the test ae_fp_greater(x,(double)(170)) - the function returns zero as expected when x>738.527209850, with a double float precision (64 bits). Convergence is reached after 5 iterations. The test with MAXLOG = 738.527209850 would even save them... NB : always prefer exp(-x) to 1/exp(x) for large values of x, to avoid any overflow error. |
|
| Author: | Fred5962 [ Sun Sep 08, 2024 12:40 pm ] |
| Post subject: | Re: Unexpected result for exponentialintegralen function |
Strange experience with this AlgLib forum, since after one month, I had zero interaction with the community, even with the administrator. The issue that I have detected and located is no more and no less than a bug. I have brought a reasonable solution with a technical reference to a robust library. Will this be fixed in a future version? |
|
| Page 1 of 1 | All times are UTC |
| Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group http://www.phpbb.com/ |
|