Determination of Exchange Parameters for CaV_4O_9
  Magnetic susceptibility measured for CaV$_4$O$_9$ is analyzed by a method with high temperature expansion. The analysis is consistent with the $g$-value which is close to 2 and is observed by an ESR experiment. Four exchange parameters in a two-dimensional Heisenberg model are determined to represent CaV$_4$O$_9$. All the exchange parameters are about 500 K and strongly frustrated. The observed spin gap originates from the frustration. 
Ken'ichi Takano and K. Sano