127 is a prime number, then among the following perfect number is 

$127=128-1=2^7-\mathrm{1....}$

We know that if $2^k-1$ is a prime number then $2^{k-1}(2^K-1)$ is a perfect number

$\therefore k=7$

Perfect number

$\begin{array}{l}=2^{k-1}(2^K-1)=2^{7-1}(2^7-1)\\ =2^6(128-1)=64\times 127\\ =8128\end{array}$