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The creation operator $\:\hat a^\dagger_i\:$ increases the occupation number of the level $\:i\:$ by $\:1\:$. If the state $\:i\:$ was empty then [see equations \eqref{ft-01}, \eqref{ft-02} in the Footnote]
\begin{align}
&\hat a^\dagger_i\vra{n_1,n_2,\cdots,0_i,\cdots} \e
\nonumber\\
&\e \!\!\!\overbrace{\hat a^\dagger_i(\hat a^\dagger_1)^{n_1}}^{\plr{\m 1}^{n_1}(\hat a^\dagger_1)^{n_1}\hat a^\dagger_i}\!\!\!(\hat a^\dagger_2)^{n_2}\cdots(\hat a^\dagger_i)^0\cdots\vra 0
\nonumber\\
&\e \plr{\m 1}^{n_1}(\hat a^\dagger_1)^{n_1}\!\!\!\overbrace{\hat a^\dagger_i(\hat a^\dagger_2)^{n_2}}^{\plr{\m 1}^{n_2}(\hat a^\dagger_2)^{n_2}\hat a^\dagger_i}\!\!\!(\hat a^\dagger_3)^{n_3}\cdots(\hat a^\dagger_i)^0\cdots\vra 0
\nonumber\\
& \e \plr{\m 1}^{n_1\p n_2}(\hat a^\dagger_1)^{n_1}(\hat a^\dagger_2)^{n_2}\!\!\overbrace{\hat a^\dagger_i(\hat a^\dagger_3)^{n_3}}^{\plr{\m 1}^{n_3}(\hat a^\dagger_3)^{n_3}\hat a^\dagger_i}\!\!(\hat a^\dagger_4)^{n_4}\cdots(\hat a^\dagger_i)^0\cdots\vra 0
\nonumber\\
& \e \plr{\m 1}^{n_1\p n_2\p n_3}(\hat a^\dagger_1)^{n_1}(\hat a^\dagger_2)^{n_2}(\hat a^\dagger_3)^{n_3}\overbrace{\hat a^\dagger_i(\hat a^\dagger_4)^{n_4}}^{\plr{\m 1}^{n_4}\cdots}\cdots(\hat a^\dagger_i)^0\cdots\vra 0
\nonumber\\
&\e \plr{\m 1}^{n_1\p n_2\p n_3\p n_4}\cdots\cdots
\tl{01}
\end{align}
that is
\begin{align}
\hat a^\dagger_i\vra{n_1,n_2,\cdots,0_i,\cdots}&\e\plr{\m 1}^{\sum\limits_{k\e 1}^{k\e i\m 1}\!\!n_k}(\hat a^\dagger_1)^{n_1}(\hat a^\dagger_2)^{n_2}\cdots(\hat a^\dagger_i)\cdots\vra 0
\nonumber\\
& \e \plr{\m 1}^{\sum\limits_{k\e 1}^{k\e i\m 1}\!\!n_k}\vra{n_1,n_2,\cdots,1_i,\cdots}
\tl{02}
\end{align}
If, on the other hand, the level $\:i\:$ was already occupied then the application of $\:\hat a^\dagger_i\:$ destroys the state vector:
\begin{equation}
\hat a^\dagger_i\vra{n_1,n_2,\cdots,1_i,\cdots} \e \plr{\m 1}^{\sum\limits_{k\e 1}^{k\e i\m 1}\!\!n_k}(\hat a^\dagger_1)^{n_1}(\hat a^\dagger_2)^{n_2}\cdots(\hat a^\dagger_i)^2\cdots\vra 0\e 0.
\tl{03}
\end{equation}
Note that \eqref{02} contains a phase factor $\:(\m 1)^{\sum\limits_{k\e 1}^{k\e i\m 1}\!\!n_k}\:$ which has no physical
significance as it depends on the labelling of the states $\:i\:$ which is arbitrary.
Footnote
Since $\:\hat a^\dagger_i\hat a^\dagger_j\e \m\hat a^\dagger_j\hat a^\dagger_i\:$ we have
\begin{align}
&\hat a^\dagger_i(\hat a^\dagger_j)^{n_j} \e \plr{\hat a^\dagger_i\hat a^\dagger_j}(\hat a^\dagger_j)^{n_j\m 1}\e \plr{\m\hat a^\dagger_j\hat a^\dagger_i}(\hat a^\dagger_j)^{n_j\m 1}\e
\nonumber\\
&\plr{\m 1}\hat a^\dagger_j\!\!\overbrace{\hat a^\dagger_i(\hat a^\dagger_j)^{n_j\m 1}}^{\plr{\m 1}\hat a^\dagger_j\hat a^\dagger_i(\hat a^\dagger_j)^{n_j\m 2}}\!\!\!\e \plr{\m 1}^2(\hat a^\dagger_j)^2\hat a^\dagger_i(\hat a^\dagger_j)^{n_j\m 2}\e \cdots\e
\nonumber\\
& \plr{\m 1}^m(\hat a^\dagger_j)^m\hat a^\dagger_i(\hat a^\dagger_j)^{n_j\m m}\e\cdots\e \plr{\m 1}^{n_j}(\hat a^\dagger_j)^{n_j}\hat a^\dagger_i
\tl{ft-01}
\end{align}
that is
\begin{equation}
\boxed{\:\:\hat a^\dagger_i(\hat a^\dagger_j)^{n_j}\e\plr{\m 1}^{n_j}(\hat a^\dagger_j)^{n_j}\hat a^\dagger_i\:\:\vp}
\tl{ft-02}
\end{equation}
i.e. the operators $\:\hat a^\dagger_i\:$ and $\:(\hat a^\dagger_j)^{n_j}\:$ commute (anticommute) if $\:n_j\:$ is even (odd).
