Formula:

The maximum likelihood of lambda is equal to minus (begin parenthesis)
the number of states divided by two (end parenthesis), times (begin
parenthesis) the natural logarithm of (begin bracket) (begin
parenthesis) one divided by the number of states (end parenthesis),
times the sum of all observations of (begin parenthesis) the
transformed absolute percentage error for a state minus the mean of
the transformed absolute percentage errors (end parenthesis) raised
to the power of two (end bracket), plus (begin parenthesis) lambda
minus one (end parenthesis), times (begin parenthesis) the sum of
the natural logarithm of the untransformed absolute percentage error
for states (end parenthesis) (end parenthesis).