Polytropic spectrum: arrangement of thermodynamic processes along the interval of the (constant) polytropic index a or ν ≡ 1/(a−1). Starting from ﹩a\to -\infty ﹩ (isochoric) and moving with increasing a (left-to-right), there are four intervals of “Explosion,” “Mild Explosion,” “Sub-adiabatic,” and “Super-adiabatic,” which are characterized by numbers of 0, 1, 2, and 3 negative terms (noted in blue) out of the three {dS/dV, dT/dV, dP/dV }, as explained by Livadiotis (2016). The examined model considers a polytropic index between isothermal and adiabatic processes, that is, in the sub-adiabatic region. (Taken from Livadiotis 2016.)