Sigma notation

1:
Fatal error: Uncaught Error: Call to undefined function get_o2() in /var/www/vhosts/ent.storkey.uk/html/inc/dynaPage.php:3542 Stack trace: #0 /var/www/vhosts/ent.storkey.uk/html/inc/dynaPage.php(3479): questionm->__construct('sigmanotques1', Array, '') #1 /var/www/vhosts/ent.storkey.uk/html/inc/dynaPage.php(718): exercisem->write() #2 /var/www/vhosts/ent.storkey.uk/html/inc/dynaPage.php(179): mainPanel->write('main-panel') #3 /var/www/vhosts/ent.storkey.uk/html/index.php(21): dynaPage->write() #4 {main} thrown in /var/www/vhosts/ent.storkey.uk/html/inc/dynaPage.php on line 3542