- #1
L. de Pudo
- 3
- 0
Hi, fellow physicists (to be). This is my first post on the forum, so I hope I get it right. If not so, please let me know :)
introduction to the problem
At the moment I am working on my physics bachelor's thesis at the theoretical department of my university (Amsterdam). My thesis focusses on a certain supersymmetric 1D lattice model, on which spinless fermions can be placed. While working in Mathematica, I found an exact expression for the occupation of sub-levels on the chain of length l (l being the number of sites on the lattice). The expression I found (f(l)), however, is a product over indices, which is not very insightful. My supervisor has asked me to rewrite this (f(l)) into a power of l. To do this, he said that I should use Stirling's approximation.
The problem
The function I found is
\begin{equation}
f(l) = \frac{2}{5} {(-1)}^l \prod_{i=0}^{l-3} \frac{3(l-i)-2}{3(l-i)-1}.
\end{equation}
While being not skilled in rewriting this, Mathematica rewrote f(l) in terms of Gamma functions. leaving out the pre-factors, I am left with
\begin{equation}
f(l)={(-1)}^l \frac{3l-2}{3l-1} \frac{\Gamma(\frac{4}{3}-l)}{\Gamma(\frac{5}{3}-l)}.
\end{equation}
On this expression I wanted to use Sterling's approximation for Gamma functions:
\begin{equation}
\Gamma(z)=\sqrt{\frac{2π}{z}}(\frac{z}{e})^{z}.
\end{equation}
The trouble that arises is that this approximation is only valid for positive arguments z of Gamma.
Attempt to Solve
To solve this, I tried to use the recursion relation
\begin{equation}
\Gamma(z)=\frac{\Gamma(z+1)}{z}
\end{equation}
If I use this recursion relation, however, I'm back at the indexed product I started with.
Does anyone on this forum know of a different approach, some sparks of creativity or otherwise good tips? All help is very welcome.
greetings,
Ludo
introduction to the problem
At the moment I am working on my physics bachelor's thesis at the theoretical department of my university (Amsterdam). My thesis focusses on a certain supersymmetric 1D lattice model, on which spinless fermions can be placed. While working in Mathematica, I found an exact expression for the occupation of sub-levels on the chain of length l (l being the number of sites on the lattice). The expression I found (f(l)), however, is a product over indices, which is not very insightful. My supervisor has asked me to rewrite this (f(l)) into a power of l. To do this, he said that I should use Stirling's approximation.
The problem
The function I found is
\begin{equation}
f(l) = \frac{2}{5} {(-1)}^l \prod_{i=0}^{l-3} \frac{3(l-i)-2}{3(l-i)-1}.
\end{equation}
While being not skilled in rewriting this, Mathematica rewrote f(l) in terms of Gamma functions. leaving out the pre-factors, I am left with
\begin{equation}
f(l)={(-1)}^l \frac{3l-2}{3l-1} \frac{\Gamma(\frac{4}{3}-l)}{\Gamma(\frac{5}{3}-l)}.
\end{equation}
On this expression I wanted to use Sterling's approximation for Gamma functions:
\begin{equation}
\Gamma(z)=\sqrt{\frac{2π}{z}}(\frac{z}{e})^{z}.
\end{equation}
The trouble that arises is that this approximation is only valid for positive arguments z of Gamma.
Attempt to Solve
To solve this, I tried to use the recursion relation
\begin{equation}
\Gamma(z)=\frac{\Gamma(z+1)}{z}
\end{equation}
If I use this recursion relation, however, I'm back at the indexed product I started with.
Does anyone on this forum know of a different approach, some sparks of creativity or otherwise good tips? All help is very welcome.
greetings,
Ludo