Let K be a field and K[ 1 , . . . , ] be a multivariate polynomial ring. Given a sequence of polynomials = ( 1 , . . . , ) and a polynomial in K[ 1 , . . . , ] with < , we consider the problem of computing the set ( , ) of points at which vanishes and the Jacobian matrix of , with respect to 1 , . . . , does not have full rank. This plays an essential role in many application areas.In this paper we focus on an important class of polynomials, the ones invariant under the action of the signed symmetric group . We introduce a notion called hyperoctahedral representation to represent -invariant sets. We study the invariance properties of the input to split ( , ) according to the orbits of and then we design an algorithm whose output being a hyperoctahedral representation of ( , ). The runtime of our algorithm is polynomial in the total number of points described by the output.