# Should "What's the Missing Number in the Series X" be allowed?

The questions can have multiple correct answers that are not accepted as they're not what OP had in mind. The "correct" answer is also very often opinionated.

Should these questions be on topic?

• I used to troll such questions on other puzzle sites to prove how absurd they are, by always answering pi/2 and when questioned (or told that I'm wrong), coming up with a function which does indeed end up with that result while fulfilling all the constraints in the puzzle.
– vsz
Mar 21 '16 at 7:25

One problem with them is that you can make a serious case that any answer to these problems is valid.

Mathematically, what these problems are asking is, there is a function $f : x \rightarrow y$ that is unknown to you, and you are given some inputs $x$, corresponding outputs $y$, and whether the examples are correct or not. The thing is, $f$ could be literally anything!

Let's say our examples are

x | y
1 | 2
3 | 4
5 | 6


and we have to guess what answer matches $x = 7$. We can construct any function of the form

f(x) = { x=1 : 2
x=3 : 4
x=5 : 6
else: z }


and have $z$ be whatever we want, whether $8$, $8,000,000$, $log(pi^e)$, or John Skeet's current reputation, and it is a mathematically valid solution to the puzzle.

This means that, without some additional restriction limiting what types of functions are possible, these questions will always run afoul of the rule that questions should be demonstrably correct.

• Normally I'd say "give the generic answer," but in this case, the generic answer is "a function $f$ that maps $x\to y$ for each $(x,y)$," which really says very little and isn't helpful. Even restricting the solution set to something like $\mathcal{P}^n$ doesn't help much (though admittedly it helps a little).
– user20
Nov 20 '14 at 0:14

I'm not sure if that policy is still applicable, however, considering its lack of enforcement these days.

Today I've gone and voted to close a bunch of questions that clearly run afoul of this policy (I ran out of close votes for the day, I may continue tomorrow). I'll see how these fare in the review queue.

• I got to admit... I've posted 1 or 2 of these, but that was before I thought it might be off topic. Nov 16 '14 at 12:13