Murasugi sum

The union of a finite sequence of surfaces taken over a disk common to every adjacent pair of the sequence, such that the boundary of the disk comprises some even number of closed arcs, disjoint except at their endpoints, which are alternatingly subarcs of the two surfaces' boundaries.

Japanese Meaning

Murasugi sumとは、隣接する各面が共通の円盤（ディスク）を介して合成された面の和集合のことを指します。この円盤の境界は閉じた弧が偶数個連なったもので、その弧は端点以外では互いに交わらず、各弧が交互に隣り合う2つの面の境界線に属するという特徴があります。