# Computing the Integral $\int r^2 \text{J}_0(\alpha r) \text{I}_1(\beta r)\text{d}r$

I encountered the following integral in a physical problem $$I=\int r^2 \text{J}_0(\alpha r) \text{I}_1(\beta r)\text{d}r$$ where $\text{J}_0$ is the Bessel function of first kind of order $0$ and $\text{I}_1$ is the modified Bessel function of order $1$. Also, $\alpha$ and $\beta$ are arbitrary real numbers.

It seems that MAPLE and WOLFRAM are not able to find the primitive. However, I think that there should be a tidy one in terms of Bessel functions.

My Thought

My instinct in integration tells me to use integration by parts and the recursive relations for Bessel functions. But I couldn't get any where yet.

• u might find this site quiet helful: dlmf.nist.gov/10.22 – tired Dec 12 '15 at 15:44
• @tired: Thanks, how can I search this integral there!? :) – H. R. Dec 12 '15 at 15:46
• i just say that u might something useful there. I guess there is a closed form but im not sure. – tired Dec 12 '15 at 16:14
• @tired: The most similar one there is the relation $10.22.7$. But they have the same multiplier $a$ in the argument! :) – H. R. Dec 12 '15 at 16:16

First note that

$$r\text{I}_1(\beta r)=\frac{\partial}{\partial\beta} \text{I}_0\left(\beta r\right).$$

Then we can write \begin{align} \mathcal{I}(t):=&\int_0^t r^2 \text{J}_0\left(\alpha r\right)\text{I}_1\left(\beta r\right) dr\\ =&\frac{\partial}{\partial\beta}\int_0^t r\text{J}_0\left(\alpha r\right)\text{I}_0\left(\beta r\right)dr\\ =& \frac{\partial}{\partial\beta}\left[\frac{t\left(\alpha \text{J}_1(\alpha t)\text{I}_0(\beta t)+\beta \text{J}_0(\alpha t)\text{I}_1(\beta t)\right)}{\alpha^2+\beta^2}\right]. \end{align} The last line follows from the formula 1.11.5.2 in Vol. II of Prudnikov-Brychkov-Marychev (and can of course be verified by direct differentiation).

• (+1) Oh, GOD! That was GREAT! :) I would be happier if you included the computation of the last integral! :) – H. R. Dec 12 '15 at 21:13
• @H.R. For the proof of the last integral, one only needs to differentiate the expression in square brackets with respect to $t$ and use recursion relations for Bessel functions. However if the answer were unknown, it would be unclear to me how to proceed. – Start wearing purple Dec 12 '15 at 21:21
• Of course, taking derivative is much easier than finding primitive! :) – H. R. Dec 12 '15 at 21:24

According to the Start Wearing Purple's answer, I just took the computations and wanted to put the final answer here

\eqalign{ & I = {1 \over {{\alpha ^2} + {\beta ^2}}}\,\,\,\,\left( {\alpha {\text{I}_1}\left( {\beta r} \right){\text{J}_1}\left( {\alpha r} \right) + \beta {\text{I}_0}\left( {\beta r} \right){\text{J}_0}\left( {\alpha r} \right)} \right){r^2} \cr & \,\,\, - {{2\beta } \over {{{\left( {{\alpha ^2} + {\beta ^2}} \right)}^2}}}\left( {\alpha {\text{I}_0}\left( {\beta r} \right){\text{J}_1}\left( {\alpha r} \right) + \beta {\text{I}_1}\left( {\beta r} \right){\text{J}_0}\left( {\alpha r} \right)} \right)r \cr}

Hope that it may be useful for future readers. There may still be other heuristic ways to compute this integral so do not hesitate to write a new answer with a different approach. :)