# Use Mathematica to determine the falling law

We have a one-variable equation $\rho(R)$ where

ρ = (14656.4+277.526*R^2)/(45.9225+R^2)^{5/2} + 0.370036/(R*(0.25+R)^3)


This equations describes the evolution of a density distribution, and as the following plot shows, the value of $\rho$ decreases rapidly with increasing $R$.

Usually density distributions drop following a law $R^{-k}$. So, is it possible to use Mathematica in order to determine the falling law at large radii $R$ in this case? In other words, to determine the values of $k$.

EDIT

Following @Vitaliy Kaurov solution I got the desired value of $k$. At the following plot, the red line corresponds to $277.526/R^3$ and as we can see it fully coincides with $\rho(R)$.

• One approach is to approximate your function for large R (say 10<R<50) by a straight line. The slope of this line will be k. Or do you have something else in mind? – bill s Aug 23 '13 at 15:50
• @bills Let's try this approach! How could we compute the slope of this line? Maybe posting a detailed answer rather than a comment. – Vaggelis_Z Aug 23 '13 at 16:14

==== Asymptotic behavior ====

\[Rho][R_] := 0.370036/(R (0.25 + R)^3) +
(14656.4 + 277.526 R^2)/(45.9225 + R^2)^(5/2)


Series[\[Rho][R], {R, Infinity, 5}]


This is obvious from the equation, but you can also see correction terms from the next orders of expansion.

==== Finite range behavior ====

If you are curious in a finite range - say from 20 to 50 - what would be the best law to describe the behavior? Then you could to a simple trick of fitting:

data = Table[{R, \[Rho][R]}, {R, 20, 50}];
model = a /x^k;
fit = FindFit[data, model, {a, k}, x]


{a -> 145.639, k -> 2.83169}

And to see how well this law fits:

modelf = Function[{t}, Evaluate[model /. fit]];
Plot[modelf[x], {x, 20, 50}, Epilog -> {Red, Point[data]}]
`

• Yes, it's working fine; see my EDIT. BTW, how did you enclose the first term with red dashed line? – Vaggelis_Z Aug 23 '13 at 17:15
• I added some other approach. For dashed line... Select cell of expression and go Top Menu >> Cell >> Convert to >> Bitmap. Then press CTRL-t for drawing tools. – Vitaliy Kaurov Aug 23 '13 at 17:26
• ctrl-t ==> ctrl-D ?? – Sjoerd C. de Vries Aug 24 '13 at 17:23
• @SjoerdC.deVries I am on Mac ;) And I think yes on Win it is CTRL-D - thanks for catching this. – Vitaliy Kaurov Aug 24 '13 at 17:57