A multi-parametric version of the nonadditive entropy
Sq is introduced. This new entropic form, denoted by
Sa,b,r, possesses many interesting statistical properties, and it reduces to the entropy
Sq for
b=0,
a=r:=1−q (hence Boltzmann–Gibbs entropy
SBG for
b=0,
a=r→0). The construction of the entropy
Sa,b,r is based on a general group-theoretical approach recently proposed by one of us,
Tempesta (2016). Indeed, essentially all the properties of this new entropy are obtained as a consequence of the existence of a rational group law, which expresses the structure of
Sa,b,r with respect to the composition of statistically independent subsystems. Depending on the choice of the parameters, the entropy
Sa,b,r can be used to cover a wide range of physical situations, in which the measure of the accessible phase space increases say exponentially with the number of particles
N of the system, or even stabilizes, by increasing
N, to a limiting value.
This paves the way to the use of this entropy in contexts where the size of the phase space does not increase as fast as the number of its constituting particles (or subsystems) increases.