## How can I as middleman verify whether a phishing site is valid if the scam listens only on the referrer link and blocks any other access methods?

How can I as a trusted user of a middleman company (such as PhishTank) verify whether a phishing site is valid if the scam listens only on a unique referrer link(randomly created) and is blocking any other access methods?

To throw a threat scenario into scene.

An attacker sent an email to a local bank officer, the email looks very similar to a official email of an employee in their company at a higher tier and the time was planned. Later they detect it was a spear-phishing attack from an old employee. They report the attack on PhishTank (for example), but there they can’t verify it because the link doesn’t allow direct access (only with a unique referrer as in the email). How can they still verify whether it was a valid report of not?

Now the real question,

On a technical view, how does such an attack work?

## How do I check whether a package from multiverse is installed?

I am on an Ubuntu system that has the multiverse repository enabled, and I’d like to see whether I can disable it.

How can I check whether any package from multiverse is installed on the system?

## How do we determine whether a heuristic is better than another in A* search Algorithm?

I am trying to solve a Maze puzzle using the A* algorithm. I am trying to analyze the algorithm based on different applicable heuristics.

Currently, I explored Manhattan and Euclidean distances. Which other heuristics are available? How do we compare them? How do we know whether a heuristic is better than another?

## Whether following language is linear or not?

I have a language $$L= \{a^nb^nc^m : n, m \ge 0\}$$.

Now, I wanted to determine whether this language is linear or not.

So, I came up with this grammar:

$$S \rightarrow A\thinspace|\thinspace Sc$$

$$A \rightarrow aAb \thinspace | \thinspace \lambda$$

I’m pretty sure(not completely however) that this grammar is linear and consequently language too is linear.

Now, when I use pumping lemma of linear languages with $$w$$, $$v$$ and $$u$$ chosen as follow I find that this language is not linear.

$$w = a^nb^nc^m, \space v = a^k, \space y=c^k$$

$$w_0 = a^{n-k}b^nc^{n-k}$$

now, $$w_0 \notin L \space (\because n_a \neq n_b)$$

So, I’m unable to find whether the language is linear or not and what goes wrong in above logic with either case. Please help.

## What Is the Complexity Class of Deciding Whether a Problem Is in NP? Is It Decidable?

Title says it all, but to clarify:

Define a problem, called $$IsInNP$$, as follows:

Given a Turing Machine $$M$$, $$IsInNP$$ is the problem of deciding if the problem that $$M$$ decides is in $$NP$$.

What is the complexity class of $$IsInNP$$? Is it even decidable? Is the answer the same for any other complexity class, like $$NP$$-hard? And are those questions even sensible to ask?

By the way, I am aware that the class $$NP$$ is not enumerable, but since I do not quite understand enumerability and it seems that recursively enumerable problems can be decidable, I do not know if that means that deciding whether a problem is in $$NP$$, or any other complexity class, is decidable.

Also, I am aware of Rice’s Theorem, and I believe it can be interpreted as saying that deciding whether a problem is in $$NP$$ is undecidable, but I am not certain.

Bonus question if the above questions are sensible: given a property $$S$$ that only $$NP$$ problems possess, does the above also mean that deciding whether a problem decided by a Turing Machine $$M_2$$ has property $$S$$ is in the same complexity class as $$IsInNP$$?

## Whether the given language is a CFL or not?

Let $$L$$ be a language defined over $$\Sigma = \left \{ a, b \right \}$$ such that $$L = \left \{ x\#y \mid x,y \in \Sigma^*, \# \text { is a constant and } x \neq y \right \}$$ State whether the language L is a CFL or not? Give valid reasons for the same.

Now, I think that the given language is not a CFL. I have used the pumping lemma test for showing that L is not CFL. Specifically, I have done the following-

Consider a string $$w = abb\#aab$$. Obviously, $$w \in L$$.

Let, $$u = \epsilon \ v = a \ w = bb\#aa \ x = b \ y = \epsilon$$

Here, $$|vx| \geq 1$$

But, $$uv^2wx^2y = aabb\#aabb \notin L$$ Therefore, pumping lemma test result is negative. Therefore, we can conclude that the given language is not a CFL.

Now, I have a doubt regarding the above method- I know that given a CFL, if we want to perform the pumping lemma test for the CFL, we must always use strings which are of length greater than or equal to the minimum pumping length. In fact, this also confirms to the condition that the length of the string $$w$$ used for the pumping lemma test (denoted by $$|w|$$) must be greater than or equal to n.

Therefore, when I use $$w = abb\#aab$$ for doing the pumping lemma test, I implicitly make the assumption that 7 is greater than or equal to the minimum pumping length (if $$L$$ were to be a CFL). Am I correct or incorrect in doing so?

## Proving whether an input sequence satisfies a given RE language

I’ve learned this a few years ago that this is impossible unless one simply ‘executes’ (in a modern computing sense) the input with the language rules, but I have some problems in just using this statement.

• The fundamental doubt is that the statement itself is well-stated. If I’m using the term ‘execution’ to describe the act of matching the rules one input element by one, is this statement valid?

• Is this statement (deciding whether an input sequence is following a language is impossible without an execution) not exactly limited to RE? In other words, I wonder this statement also holds even for the languages in other classes.

• I’m not even sure how I can search for this statement and confirm from the external source.

(By RE, here I indicate the recursively enumerable languages, not the regular expression)

## Checking whether the win-loss standings of a league are possible

You’re hosting a 1 v 1 basketball league with a game schedule. At the end of the league you have each player report their supposed win-loss record (there are no ties), but you want to check whether the proposed standings were actually possible given the schedule.

For example: you have four players (Alice+Bob+Carol+Dave) and your schedule is a simple round robin. The reported standings [A: 3-0 B: 1-2 C: 1-2 D: 1-2] and [A: 2-1 B: 1-2 C: 1-2 D: 2-1] would be possible, but the standing [A: 3-0 B: 0-3 C: 0-3 D: 3-0] would not be.

Now suppose the schedule is instead a 3 game head to head between Alice+Bob and Carol+Dave. The reported standing [A: 3-0 B: 0-3 C: 0-3 D: 3-0] is now possible, but [A: 3-0 B: 1-2 C: 1-2 D: 1-2] would no longer be.

(The schedule does not need to be symmetric in any way. You could have Alice only play against Bob 10 times, then make Bob+Carol+Dave play 58 round robins against each other.)

Problem: Given a schedule with k participants and n total games, efficiently check whether a proposed win-loss standings could actually occur from that schedule.

The O($$2^n$$) brute force method is obvious, enumerate all possible game outcomes and see if any match the proposed standings. And if k is small increasing n doesn’t add much complexity – it’s very easy to check a two person league’s standings regardless of whether they play ten games or ten billion games. Beyond that I haven’t made much headway in finding a better method, and was curious if anyone had seen a similar problem before.

## Is there any lore about whether a creature can see themselves and their gear while invisible?

I’ve separately asked about the mechanics of whether an invisible creature can see themselves and their own gear in D&D 5E. And from a rules perspective, they can’t. But from a more flavor perspective, I want to know if there’s any indication in the various D&D published materials (maybe in novels, movies, articles, or setting sourcebooks for prior editions) whether or not people can see themselves (and their things) when invisible. Does somewhere explain just what a character experiences when they look at themselves while invisible, like saying whether they’re surprised by seeing the ground through their own feet, or whether there’s some ghostly effect where they can tell they’re invisible to others but can see themselves to some extent?

## External links: Whether & how to distinguishing them from internal links, and to open them

How do other IAs/UXD’s treat external links & what are the perceived pros/cons of different options. Specifically looking for recommendations on:

• whether & how to visually distinguish external links from internal links
• whether to open them in the same window/tab or a different window or tab

I’ve found some great feedback re these questions on http://www.ixda.org/search.php?tag=external+links & http://www.useit.com/alertbox/open_new_windows.html

Looking for additional opinions, thoughts & especially any usability test findings contributors of this site might have re these questions.