Asymptotic of incomplete Gamma function, $\Gamma(n,n/a)$ Let $a>1$. I need to approximate (for n large)
$$
\Gamma(n,n/a) \approx f(n)
$$
Approximations I found are for the case which the second argument is fixed. Is there any simple formula for this asymptotic?

$\Gamma(n, n/a)$ and $\Gamma(n)$ will become very close (in the sense that their ratio converges to $1$) as $n$ grow. The following computation provides a more precise error bound for their ratio:

I will write $\alpha = 1/a$ so that $0 < \alpha < 1$. Then
$$ \Gamma(n, \alpha n) = \Gamma(n) \biggl[ 1 - \frac{1}{\Gamma(n)} \int_{0}^{\alpha n} t^{n-1}e^{-t} \, \mathrm{d}t \biggr]. $$
Since $t \mapsto t^{n-1}e^{-t}$ is increasing on $0 \leq t \leq n-1$, for large $n$ we have
$$ \int_{0}^{\alpha n} t^{n-1}e^{-t} \, \mathrm{d}t \leq (\alpha n)^n e^{-\alpha n}. $$
So by the Stirling's approximation,
$$ \frac{1}{\Gamma(n)} \int_{0}^{\alpha n} t^{n-1}e^{-t} \, \mathrm{d}t 
\leq \frac{(\alpha n)^n e^{-\alpha n}}{\sqrt{2\pi} \, n^{n-\frac{1}{2}} e^{-n}}
= \frac{1}{\sqrt{2\pi}} n^{1/2} (\alpha e^{1-\alpha})^n .
$$
Similarly, by noting that
$$ \int_{0}^{\alpha n} t^{n-1}e^{-t} \, \mathrm{d}t
\geq \int_{\alpha n-1}^{\alpha n} t^{n-1}e^{-t} \, \mathrm{d}t
\geq (\alpha n - 1)^{n-1} e^{-\alpha n+1}, $$
we have
$$ \frac{1}{\Gamma(n)} \int_{0}^{\alpha n} t^{n-1}e^{-t} \, \mathrm{d}t \geq c_{\alpha} n^{-1/2} (\alpha e^{1-\alpha})^{n} $$
for some constant $c_{\alpha} \in (0, 1)$ depending only on $\alpha$. This tells that
$$ \Gamma(n, \alpha n) = \Gamma(n) \left(1 - n^{\Theta(1)}(\alpha e^{1-\alpha})^n \right), $$
where $\Theta(1)$ represents a bounded sequence in $n$.

Remarks. A less precise asymptotic formula $\Gamma(n, \alpha n) = \Gamma(n)(1 - o(1))$ can be obtained by applying the central limit theorem to
$$ \frac{\Gamma(n, \alpha n)}{\Gamma(n)} = \mathbf{P}(\tau_1 + \cdots + \tau_n \geq \alpha n), $$
where $\tau_k$'s are independent $\operatorname{Exp}(1)$ variables.
