Update theory_intro.jl

[WalterMadelim]

Your cdot notation is not correct.

e.g. we want to express the idea---the function x -> f(x, u) is convex for some fixed parameter u,
we can write f(\cdot, u) is convex tersely.

Here the \cdot means "whatever"---much of in julia, x -> 2x === y -> 2y (whatever the symbol for the local variable)

[odow]

Let's just be explicit. Clever math notation is confusing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.37%. Comparing base (5a0433f) to head (dbb377b).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #886      +/-   ##
==========================================
+ Coverage   96.32%   96.37%   +0.04%     
==========================================
  Files          28       28              
  Lines        4032     4032              
==========================================
+ Hits         3884     3886       +2     
+ Misses        148      146       -2     

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