N=64 j=?

It seems like you're starting to set up a problem, but I need a bit more context to provide a meaningful answer. The variable "N" is given as 64, but "j" could stand for many different concepts depending on the context (e.g., an index in a series, a variable in a formula, etc.).

Could you please provide more details about the context in which you are trying to find "j"? For example, are you dealing with a mathematical equation, a statistical analysis, or something else?