$\"\"$

\

The function is $\"\"$ .

\

Take out the constant term $\"\"$ .

\

$\"\"$

\

$\"\"$

\

$\"\"$

\

Apply Summation formula $\"\"$ .

\

Apply Summation formula $\"\"$ .

\

Apply Summation formula $\"\"$ .

\

$\"\"$

\

$\"\"$ .

\

$\"\"$

\

$\"\"$ As $\"\"$ .

\

$\"\"$

\

$\"\"$

\

$\"\"$ .

\

$\"\"$

\

$\"\"$ .