  Let L_n be the n-dimensional Lorentz cone. A linear map M from R^m to R^n is
called Lorentz-positive if M[L_m] is contained in L_n. We extend the notion of
concurrence, which was initially introduced to quantify the entanglement of
bipartite density matrices, to Lorentz-positive maps and provide an explicite
formula for it. This allows us to obtain formulae for the concurrence of
arbitrary positive operators taking 2 x 2 complex hermitian matrices as input
and consequently of arbitrary bipartite density matrices of rank 2. Namely, let
P: H(2) \to H(d) be a positive operator, and let \lambda_1,...,\lambda_4 be the
generalized eigenvalues of the pencil \sigma_2(P(X)) - \lambda det X, in
decreasing order, where \sigma_2 is the second symmetric function of the
spectrum. Then the concurrence is given by the expression C(P;X) =
2\sqrt{\sigma_2(P(X)) - \lambda_2 det X}. As an application, we compute the
concurrences of the density matrices of all graphs with 2 edges. Similar
results apply for a function which we call I-fidelity, with the second largest
generalized eigenvalue \lambda_2 replaced by the smallest eigenvalue \lambda_4.
