In this talk we'll discuss the construction of certain indefinite theta functions involving the Niemeier lattice $A_1^{24}$. We'll relate these indefinite theta functions to the McKay--Thompson series attached to the Mathieu group $M_{24}$
Links:
[1] https://www.mpim-bonn.mpg.de/taxonomy/term/39
[2] https://www.mpim-bonn.mpg.de/node/3444
[3] https://www.mpim-bonn.mpg.de/node/7866