# A simple question: Input problem

I have a simple problem. First I define the value of variables, then I calculate the expression as the following:

Code 1:

a = 10^-6;
b = 10^-3;
c = 1;
d = 0.1;
s = -d Sqrt[b^2 + c^2];

Integrate[(w E^(-w/a))/(w - s), {w, 0, ∞}]
-3.744368975478385*10^43415


As you see the result is so large. But if I manually insert the value of s (-0.1) into the expression, I would get zero, which is is close to the correct answer:

Code 2:

a = 10^-6;
b = 10^-3;
c = 1;
d = 0.1;
s = -d Sqrt[b^2 + c^2];

Integrate[(w E^(-w/a))/(w + 0.1), {w, 0, ∞}]
0.


What is the problem with the first code?

• Have you tried using exact numbers instead of approximate ones? In other words, does the result of your first calculation change if you set d = 1/10 instead? Also, the exact value of s is very close to $-0.1$, but not exactly equal to that value. Why do you think yours is a valid substitution? Jan 26, 2016 at 6:35
• Yes. If I use d=1/10 instead, the result would be different. It is strange, because the result is 9.99979*10^-12, not zero as code 2. I know that 0.1 is different from 1/10, but it is improbable that their difference leads to a number of the order of 10^43415 (code 1). Moreover, I substitute the mathematica value of s into code 2. I still don't understand why the result of definition (code 1) is very different from the result of manual substitution (code 2). Jan 26, 2016 at 7:05
• But the numerical zero does not have to be the right answer. The number you get for d = 1/10 is fine. In code 2 may be the integration to infinity gets a different value for s=-0.1, which is not the number for d=1/10 (as already pointed out by MarcoB). Jan 26, 2016 at 7:09
• While the handling of inexact input by Mathematica's exact solvers has improved recently, I feel one should still use caution when doing it. Use NIntegrate, or if possible, solve exactly before plugging in inexact parameters (e.g., this works in the present case Block[{s}, Integrate[(w E^(-w/a))/(w - s), {w, 0, \[Infinity]}, Assumptions -> s < 0]]). Jan 27, 2016 at 12:42

a = 10^-6;
b = 10^-3;
c = 1;
d = 0.1;
s = -d Sqrt[b^2 + c^2]


-0.1

this result is approximated for display. You can see the complete result by placing your cursor in front of the -0.1 and pressing the space bar.

Alternatively

InputForm[s]


-0.1000000499999875

Edit

With s = -d Sqrt[b^2 + c^2] the integral calculation yields ...

in Mathematica 7.0.1.0 : 9.99979500615538*10^-12

in Mathematica 10.3.1 : -3.744368975619640*10^43415

• Yes. That 's right. But this tiny difference would result in a difference of the order of 10^43415 in the final result? Jan 26, 2016 at 13:41
• In Mathematica 7.0.1.0 the result is 9.99979500615538*10^-12. However, with s = -0.1 the result is 9.999800005898997*10^-12 which is 4.99974*10^-18 more. Jan 26, 2016 at 13:47
• No. For d=0.1, I get -3.7*10^43415. Strangely, for d=1/10, I get your result, which is 9.9998*10^-12. Why such a difference in the result? Jan 26, 2016 at 14:04
• On Mathematica 10.3.1 the result is -3.744368975619640*10^43415 but not on Mathematica 7.0.1.0 (both running on Windows 10). Jan 26, 2016 at 14:07
• @ChirsDegnen Thanks. I have a bunch of expressions. I can't substitute the variables manually. It seems that I should go to Mathematica 7. Jan 26, 2016 at 14:23

To sum up what we have been saying in comments, it may sometimes be dangerous to use symbolic solvers (e.g. Integrate) with inexact input (e.g. $d=0.1$). It is better in your case to evaluate the integral symbolically, and then calculate the approximate numerical value:

a = 10^-6;
b = 10^-3;
c = 1;
s = -d Sqrt[b^2 + c^2];
d = 1/10;

Integrate[(w E^(-w/a))/(w - s), {w, 0, Infinity}]


N[%]
(* Out: 9.99979*10^-12 *)


The result is of course very close to zero, but not exactly zero. The problem doesn't lie in how close $s$ is to $-0.1$, but in whether you use exact or approximate input to Integrate, which the function does not handle gracefully in this case.

• +1 Also works with NIntegrate[(w E^(-w/a))/(w - s), {w, 0, Infinity}, AccuracyGoal -> 5]  Jan 28, 2016 at 14:57