Why do Upstart jobs use `start on runlevel [2345]`?

Yes they are used for compatibility with other distributions and the LSB PDA specifications. I have had multiple systems in which runlevels 3,4,5 were used, but don't ask me what they were, this was years back.

If it bothers you, it shouldn't be an issue to remove those runlevels in all the configurations, but I would recommend leaving it as is. If it ain't broke, don't fix it.

If I compile Firefox instead of binary install it will work faster? [closed]

How Ubuntu plans to support PHP7?

On a subclass of kernel functions

Evaluate $\lim\limits_{x \to 0} \left(\frac{x}{\sin x }\right)^{1/x^2}$.

Writing $k(x_1+\frac{x_2+1}{2k})^2+\frac{4k^2-1}{4k}x_2^2+\frac{2k-1}{2k}x_2-\frac{4k^2+1}{4k}$ in the form $A(x_1+Bx_2+C)^2+E(x_2+F)^2+G$

Positive operator on a complex Hilbert space is symmetric?

Which of these, if any, is more correct unit vector notation?

Spivak, Ch. 4 Graphs, Problem 15: Draw the graph of $f(x)=ax^2+bx+c$?

If $-2\leq f(x)\leq 4$ for all real $x$, then which must exist? $\lim_{x\to-2}\frac1{f(x)}$, $\lim_{x\to-2}\frac{f(x)}x$, $\lim_{x\to-2}(|f(x)|-f(x))$

Expected number of cuts before a stick is halved

How to find the "relative" defining polynomial of an extension of number fields?

Complex integral $\int \frac{e^{iz}}{(z^2 + 1)^2}\,dz$ with Cauchy's Integral Formula.