If it is a mathematical paper it is targeted at other mathematicans eho should be able to understqnd the notation.

My math professors would teach maths under the presumption we knew mathematical notation. I felt I got thrown in the deep in during first-year calculus units but I also now feel strangely adept at reading maths so it had a positive effect.

the other problem is assuming the reader knows nothing, going too deep into the foundations supporting the conclusion.

mathematical knowledge is like scaffolding. a reader has to gradually build up and buttress the lower layers before attempting to move up. skipping steps does no one any favours.