\BOOKMARK [1][-]{section.1}{Introduction}{}% 1
\BOOKMARK [1][-]{section.2}{A recursive definition for Spn}{}% 2
\BOOKMARK [1][-]{section.3}{An identity involving Bernoulli numbers}{}% 3
\BOOKMARK [1][-]{section.4}{Symmetry of Spn}{}% 4
\BOOKMARK [1][-]{section.5}{Open problems}{}% 5
\BOOKMARK [1][-]{section.6}{Acknowledgment}{}% 6