integrating exponential How can you integrate $\frac{e^{x-3}}{x^4} dx$.
I tried integration by parts but it's not possible.
Is some substitution possible ? 
I started off solving the diff eqn $(xy^2 + 3e^{x-3})dx - x^2ydy = 0$
It's a non-exact first order equation whose integrating factor is $1/x^4$.
Finally I got to the equation where I needed to integrate $\frac{e^{x-3}}{x^4} dx.$
But I can't seem to proceed.


I tried integration by parts but it's not possible.

Actually it is, you just have to do it repeatedly, $\big($three or four times$\big)$, with regards to $\dfrac1{x^n}$, and then lastly employ the exponential integral. You should finally be able to arrive at $\dfrac1{6~e^3}\bigg[\text{Ei}(x)-e^x\cdot$ $\cdot\dfrac{x^2+x+2}{x^3}\bigg]$
