Analytically computable tangle for three-qubit mixed states

We present a new tripartite entanglement measure for three-qubit mixed states. The new measure $t_{\mathrm{r}}(\rho)$, which we refer to as the r-tangle, is given as a kind of the tangle, but has a feature which the tangle does not have; if we can derive an analytical form of $t_{\mathrm{r}}(\rho)$ for a three-qubit mixed state $\rho$, we can also derive $t_{\mathrm{r}}(\rho')$ analytically for any states $\rho'$ which are SLOCC-equivalent to the state $\rho$. The concurrence of two-qubit states also satisfies the feature, but the tangle does not. These facts imply that the r-tangle $t_{\mathrm{r}}$ is the appropriate three-partite counterpart of the concurrence. We also derive an analytical form of the r-tangle $t_{\mathrm{r}}$ for mixtures of a generalized GHZ state and a generalized W state, and hence for all states which are SLOCC-equivalent to them.