What is the SN in math?

Excuse me, could you please clarify what the abbreviation "SN" stands for in the context of mathematics? I'm not familiar with this particular term and I'm curious to know its meaning and application within the field. Is it related to a specific branch of mathematics, such as algebra, calculus, or geometry? Or is it a notation used in a more specialized area, like number theory or topology? Any information you could provide would be greatly appreciated.