I have studied the following recurrence. The ratio between f(n) and n^log_b(a) is log n so there is non polynomial difference but I have studied from book that it can be solved by master method.
$T (n) = 2T (n/2) + n log n$
On the other hand ratio in $T (n) = 2T (n/2) + n /log n$ is $1/log n$ so this also has non polynomial difference but it can not be solved by master method. Because of this, I am having confusion in recognizing recurrences which can be solved by master method. I know what Master method is and how it works. I just want to understand why both of above recurrences cannot be solved by master theorem.
