Proofs of Ibukiyama’s conjectures on Siegel modular forms of half-integral weight and of degree 2

Abstract

We prove Ibukiyama’s conjectures on Siegel modular forms of half-integral weight and of degree 2 by using Arthur’s multiplicity formula on the split odd special orthogonal group $${\text {SO}}_5$$ and Gan–Ichino’s multiplicity formula on the metaplectic group $${\text {Mp}}_4$$ . In the proof, the representation theory of the Jacobi groups also plays an important role.