For every natural integer $p \geqslant 1$ and every real number $x$, we set $\widetilde { A } _ { p } ( x ) = A _ { p } \left( \frac { x } { 2 \pi } - \left[ \frac { x } { 2 \pi } \right] \right)$. For $p \in \mathbb { N } ^ { * }$, deduce that $a _ { 2 p } = A _ { 2 p } ( 0 ) = ( - 1 ) ^ { p + 1 } \frac { S ( 2 p ) } { 2 ^ { 2 p - 1 } \pi ^ { 2 p } }$.