Notation in a paper I'm looking at a paper from H Fuchs called Optimal Surface Reconstruction from Planar Contours dated 1977. It can be found here. On page 2 of the PDF (stamped as page 694) the notation $+_k$ is introduced to denote addition modulo k. I understand what this means. The paper continues with this paragraph: !Extract from paper I am not fully clear when P is subscripted like in this example: !Focused extract from paper Looks like $m$ is being superscripted with a $1$. Later in the paragraph it gets a $2$. $m$ is the number of points in a contour, but why the superscripted number? I wonder whether the notation needs a $+$ operator? Hope someone can assist.

It looks to me like $P$ is subscripted with $i+_m1$, that is $i+1$ modulo $m$, and that interpretation seems to make sense in the context you've posted.