A polarized abelian variety (X,\lambda) of dimension g over a local field K determines an admissible representation of GSpin_{ 2g+1}(K). We show that the restriction of this representation to Spin_{ 2g+1}(K) is reducible if and only if X is isogenous to its twist by the quadratic unramified extension of K. When g=1 and K = Q_p, we recover the well-known fact that the admissible GL_2(K) representation attached to an elliptic curve E is reducible upon restriction to SL_2(K) if and only if E has supersingular reduction.
Categories: math.NT math.RT