# Final Notice: The Earth will fall into Null Space

Today, I received my favorite unsolicited doomsday screed from a nutter named Frank. It reads in part (I take an image to preserve its formatting — and to show it is not written in Comic Sans so I take it seriously):

Dear Frank,

No worries buddy!
The Null Space is a fine place to be.
Since it appears you specialize in a different discipline than sparse approximation,

• Replace “Objective Uncertainty Test” with “Exact Recovery Condition”. See the Testament of Tropp (Chapter “Greed is Good”).
• Replace “Hell” with “measurements in $$\mathcal{R}^M$$,” and “Earth” with “signal in $$\mathcal{R}^N$$.”
• Define “Closed System” as Earth having mostly zeros, and $$K$$ non-zeros. You can say “Earth is Sparse” if you wish.
• Define “Open System” as the set of all solutions, among which the sparsest one (fewest “mass units” in your words, I think) is what we seek.
• Replace “polarized First Space” with restricted isometry property (abbreviated RIP — but don’t look too deeply for its non-existent religious reference).
• Thus the “sequence begins”: $$\mathcal{O}(M) = K\log(N/K)$$.
• NB! Since it appears you are doing most of your eschatology in MATLAB (from the bullet points “>>”), make sure you are using “\” instead of “/”. That little mistake has tripped me up a few times in my end times predictions!
• Also, in accordance with the gospel of “reproducible research”, you should make available all your algorithms. It really does have an impact — plug and play evangelism and such.

Nonetheless, fascinating work Frank! Hope to see you in the Null Space soon.