I'm reading now about theta series and its significance in researching self-dual, even lattices. Now, I found the wikipedia article about the Leech lattice, but I'm having trouble understanding where the $\theta$ series have come from.
As I see it, the $\theta$ series of lattice of order 24, equals to a form in $M_{12} = cE_{12} + \Delta$, so the coefficient doesn't match.
Explanation will be highly appreciated.