Actually, the usual presentation of math notation in textbooks, papers, etc., while dense, does use whitespace -- and often large whitespace or line breaks for factoring out definitions used in the main equation for readability and ready comprehension. [1] Let-bindings and similar constructs in other functional languages are, in fact, largely driven by this kind of factoring-out in mathematical notation (not just in their use of whitespace, but in general.)

[1] For an example of the large whitespace for factoring-out, see the factoring out of J-sub-k on the top of p. 460 of http://titan.fsb.hr/~venovako/dist/tensor.pdf