Constraints on Dark Energy state equation with varying pivoting redshift

We assume the DE state equations w(a)=w0+wa(ap-a), and study the dependence of the constraints on w0 and wa coefficients on the pivoting redshift 1+zp=1/ap. Coefficients are fitted to data including WMAP7, SNIa (Union 2.1), BAO's (including WiggleZ and SDSS results) and H0 constraints. The fitting algorithm is CosmoMC. We find specific differences between the cases when ν-mass is allowed or disregarded. More in detail: (i) The zp value yielding uncorrelated constraints on w0 and wa is different in the two cases, holding ∼0.25 and ∼0.35, respectively. (ii) If we consider the intervals allowed to w0, we find that they shift when zp increases, in opposite directions for vanishing or allowed ν-mass. This leads to no overlap between 1σ intervals already at zp>∼0.4. (iii) The known effect that a more negative state parameter is required to allow for ν mass displays its effects on wa, rather than on w0. (iv) The w0-wa constraints found by using any pivot zp can be translated into constraints holding at a specific zp value (0 or the zp where errors are uncorrelated). When we do so, error ellipses exhibit a satisfactory overlap.