Evaluating integral with varying upper limit?

*To*: mathgroup at smc.vnet.net*Subject*: [mg71035] Evaluating integral with varying upper limit?*From*: AES <siegman at stanford.edu>*Date*: Mon, 6 Nov 2006 02:52:36 -0500 (EST)*Organization*: Stanford University

Given a function f[x] which happens to be rather messy and not analytically integrable, I want to evaluate the function g[y_] := NIntegrate[f[x], {x, ymin, y} ] with ymin fixed and ymin < y < Infinity. I suppose that FunctionInterpolate is the way to go here (???). But, are there tricks to tell FunctionInterpolate what I know in advance, namely that f[x] is everywhere positive, and decreases toward zero rapidly enough at large x that g[y] will approach a finite limiting value as y -> Infinity? (which value I'd like to have FI obtain with moderate accuracy -- meaning 3 or 4 significant digits, not 10 or 20) Thanks . . .

**Follow-Ups**:**Re: Evaluating integral with varying upper limit?***From:*Carl Woll <carlw@wolfram.com>

**Re: Evaluating integral with varying upper limit?***From:*Carl Woll <carlw@wolfram.com>

**Re: Evaluating integral with varying upper limit?***From:*"Chris Chiasson" <chris@chiasson.name>

**Re: Evaluating integral with varying upper limit?***From:*"Chris Chiasson" <chris@chiasson.name>