"Zhen Chen" <[EMAIL PROTECTED]> writes: > I have problem evaluating the expression h = exp(x)/(exp(exp(x))-1) > for large negative x. This expression is actually the probability > that y = 1 when y is a Poisson random variable truncated at 0, hence > must satisfy 0 <= h <= 1. However, when > x < -18, I may get an h value that is larger than 1 while the true > value should be a quantity that is smaller than but very close to 1. > For example when x = -19, h = 1.00000000031174. I tried to use > different form of h and none of > them give me an h value that is less than or equal to 1. I also tried > to find patterns, but discovered none: for some x < -18, h is below 1; > for others > h is greater than 1. Is there any trick that enables me to obtain > theoretically correct results from this expression?
expm1() should help... What platform is this? I don't see it until x=-30: > exp(-30) [1] 9.357623e-14 > exp(exp(-30))-1 [1] 9.348078e-14 which is of course "wrong" since exp(x) > x + 1 mathematically. However, notice that in double precision calculations the latter value is only accurate up to 1e-15 or so. -- O__ ---- Peter Dalgaard Blegdamsvej 3 c/ /'_ --- Dept. of Biostatistics 2200 Cph. N (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918 ~~~~~~~~~~ - ([EMAIL PROTECTED]) FAX: (+45) 35327907 ______________________________________________ [EMAIL PROTECTED] mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
