  In this note we study the reduction of 4d Seiberg duality to 3d for SP(2N)
SQCD with an adjoint field. We follow a general prescription that consists in
compactifying the dual 4d theories on the circle. This generates an effective
3d duality. The pure 3d duality is obtained by combining the zero radius limit
with a real mass flow. Here we perform this limit by a double scaling
procedure: we turn on real masses proportional to the radius before shrinking
the circle. We apply this mechanism to the reduction of the 4d superconformal
index to the three sphere partition function. While the reduction of the 4d
index on the circle is straightforward, the 3d limit necessitates the double
scaling. We describe this limit on the index, finding the integral identity for
the partition functions of the 3d dual theories.
