Before trying to find the location of secondary resonances, we need to have good estimates of the planetary frequencies g₅, g₆, and s₆.   We refer to the work of Bretagnon 1974 (Termes a longue period dans le systeme solaire), which gives the following values for the frequencies: g₅=0.0206383049 rad/1000 yr, g₆=0.1369257468 rad/1000yr, and s₆=-0.1277351863 rad/1000yr.  To obtain the frequencies in "/yr we first compute the periods, which are given by
 

From the periods, we can get the frequencies in "/yr using the usual formula f=360*3600/P.   This gives the following values for the planetary frequencies:

At this point, we need to find a way to compute the frequency and period associated with a suspected secondary resonance.  For example, let us consider the 2:3 resonance between the Great-Inequality and the argument of pericenter.  The resonant argument of such resonance must respect the D' Alembert rule, so the sum of the coefficients of the resonance must be zero.   In our case we have

Since \omega is equal to \varpi-\Omega, the coefficients for \omega are zero.    The terms associated with the Great-Inequality give a coefficient sum of -2*(5-2)=-6.   We need to add six*secular variable from this resonant argument.  Using the planetary frequencies we have three simple possibilities (and several other combinations whose coefficient sum is six):

These equations constrain the value of the frequency in \omega, which is equal to (2*f_GI-6*f...)/3.   From the value of the frequency of the resonance, it is very easy to determine the period of the resonance.