Douglas B. West, The grammar according to West
Kevin P. Lee, A guide to writing mathematics.
Bruce C. Berndt, How to write mathematical papers. Some of his papers.
Jerzy Trzeciak, Writing Mathematical Papers—a Few Tips.
S Maurer, Writing Mathematics Papers.
BJORN POONEN, PRACTICAL SUGGESTIONS FOR MATHEMATICAL WRITING
"by the Pythagorean Theorem", or "by Pythagoras' Theorem", or "by Pythagorean Theorem": the first two are favored.
"a unique solution" or "the unique solution"
A theorem of Euler's, a theorem due to Euler
Tim Gowers, The language and grammar of mathematics