Is there a way of approximating e^-x for large x

  • Thread starter Chain
  • Start date
In summary, the integrand is one you can anti-differentiate without much trouble, so you can get the exact answer. The exponential will drop very quick, so regions with r>1m+eps are irrelevant and r^2 is nearly constant and =1m^2. Therefore, the integral is simply 4m^2/a^3 * exp(-2r/a) which can be evaluated as 2m^2/a^2 * exp(-2m/a) ≈ 8*10^20 * exp(-4*10^10) ≈ 10^(-10^10)
  • #1
Chain
35
3
I'm trying to evaluate an integral with e^-x where x is huge in the domain of the integral so I can't evaluate it numerically without making an approximation.
 
Physics news on Phys.org
  • #2
Can you add more context? There might be a clever way to approximate e^(-x), but if that value is not added to something, e^(-x)=e^(-x-x0)*e^(x0) where the second factor is independent of x and the first factor can be chosen to be about e^0.
 
  • #3
The integral is 4*(r^2)*exp(-2*r/a)/a^3 integrated between 1 and ∞ (The probability an electron in the ground state of hydrogen is more than 1 metre away from the nucleus) a=0.529*10^-10
 
  • #4
Note that your integrand is one you can anti-differentiate without much trouble, so you can get the exact answer. (e.g. integration by parts. Or computer algebra package)
 
  • #5
If you just want an estimate: The exponential will drop very quick, so regions with r>1m+eps are irrelevant and r^2 is nearly constant and =1m^2. Therefore, the integral is simply 4m^2/a^3 * exp(-2r/a) which can be evaluated as 2m^2/a^2 * exp(-2m/a) ≈ 8*10^20 * exp(-4*10^10) ≈ 10^(-10^10) where the last approximation is very rough.
 
  • #6
e-x = 10-0.43429448190325x. The constant is log10e.

Using the above for large x, you can separate the integer and fractional parts of the exponent. I assume you know how to proceed from here.
 
  • #7
Are you talking about Gauss–Laguerre quadrature?

[tex]\int_0^\infty f(x) e^{-x} \mathop{\text{dx}}\sim \sum_{i=1}^n w_i f(x_i)[/tex]

where xi are zeros of a Laguerre polynomial and

[tex]w_i=\frac{x_i}{(n+1)^2[L_{n+1}(x_i)]^2}[/tex]
 
  • #8
Fair enough, yeah I realized after posting this the integral could be solved analytically >__< and I got a valule of something like 10^(-10^10) but thanks for the responses :)
 

FAQ: Is there a way of approximating e^-x for large x

How do you approximate e^-x for large x?

One way to approximate e^-x for large x is by using the Taylor series expansion of e^-x. This involves breaking down the function into a sum of infinitely many terms, each representing a successive derivative of the function evaluated at a particular point. By taking only a finite number of terms, we can get an approximation of e^-x that becomes more accurate as the number of terms increases.

What is the best method for approximating e^-x for large x?

The best method for approximating e^-x for large x may depend on the specific context and desired level of accuracy. In addition to using the Taylor series expansion, other methods such as the Euler-Maclaurin formula and numerical integration techniques can also be used to approximate e^-x for large x.

Is there a limit to how accurate an approximation of e^-x can be for large x?

Yes, there is a limit to how accurate an approximation of e^-x can be for large x. This is because as x becomes larger, the function e^-x approaches 0 very quickly, making it difficult to calculate accurately. Additionally, the use of finite terms in approximation methods means that there will always be some degree of error in the approximation.

Can technology be used to approximate e^-x for large x?

Yes, technology can be used to approximate e^-x for large x. Many scientific calculators and computer programs have built-in functions for calculating e^-x, and some even have options for adjusting the number of terms used in the approximation. Additionally, programming languages such as Python have libraries that allow for efficient calculation of the Taylor series expansion of e^-x.

Are there any real-world applications for approximating e^-x for large x?

Yes, there are various real-world applications for approximating e^-x for large x. One example is in the field of finance, where the exponential function is commonly used to model growth and decay. In such cases, the ability to approximate e^-x for large x can be useful in predicting the behavior of financial systems. Additionally, approximating e^-x can also be used in various scientific and engineering contexts where exponential functions arise, such as in population growth models or electrical circuits.

Similar threads

Back
Top