Gap Probabilities for Double Intervals in Hermitian Random Matrix Ensembles as $\tau$-Functions -- Spectrum Singularity case

The probability for the exclusion of eigenvalues from an interval $(-x,x)$ symmetrical about the origin for a scaled ensemble of Hermitian random matrices, where the Fredholm kernel is a type of Bessel kernel with parameter $a$ (a generalisation of the sine kernel in the bulk scaling case), is considered. It is shown that this probability is the square of a $\tau$-function, in the sense of Okamoto, for the Painlev\'e system \PIII. This then leads to a factorisation of the probability as the product of two $\tau$-functions for the Painlev\'e system \PIIIdash. A previous study has given a formula of this type but involving \PIIIdash systems with different parameters consequently implying an identity between products of $\tau$-functions or equivalently sums of Hamiltonians.