## When should we use sequential representation for binary tree?

There are two ways of representing a binary tree:

1. Sequential – usually done by an array

When should we use sequential and when linked representation considering we know how the tree’s properties?

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## Correct Turing machine representation for Rice Theorem proof

Consider the language L1. From Rice Theorem I know L1 is not decidable (i.e. undecidable).

L1 = { R(M) | R(M) is a TM and 1011 ∈ L(M)}

For example if I want to represent by diagram a TM $$M_1$$ that accepts the string 1011 and a TM $$M_2$$ that doesn’t accept the string 1011 (e.g., $$M_2$$ accepts only the empty string), following the Rice Theorem not-trivial property, I need to use a (1) acceptance by final states or (2) acceptance by halting or (3) I can use both because I know (by theorem) they are equivalent?

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## Fundamental motivation behind the use of bits and binary representation

This is a naive question, but what makes binary representation special from a theoretical standpoint and from the standpoint of information theory?

If for technical reasons building ternary computers where the information is encoded as trits was easier than building traditional binary computers, I get the feeling that most theoretical computer science and information theory would still use bits and base-2 representation by default.

Even if I have an intuitive feeling of why, I would have liked to know a formal explanation of that: from a purely theoretical standpoint what makes bits and binary representation special compared to any other base?

If the answer is more complex than one may first think, links to books and scientific papers are welcome.

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## Genetic Algorithms Representation for directed acyclic graphs

I have a problem where every possible solution is a Directed Acyclic Graph (DAG) plus if a node $$x$$ has $$d$$ incoming edges in the graph, there is $$2^d$$ binary bits associated with $$x$$ that I need also to discover.

So every possible solution is $$(G,\theta)$$. I am learning about GAs and trying to represent a solution by a binary string. The ultimate goal is to learn a hidden graph $$(G,\theta)$$ from some given data by a genetic algorithm. I am wondering what is the most compact representation of binary string for this type of solutions. The graph can be represented by $$n\times n$$ matrix $$c$$ where $$c_{ij}=1$$ iff there is an incoming edge from $$x_i$$ to $$x_j$$. But struggling with the $$\theta$$ representation.

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## Space efficient representation of Regular graphs

Let $$G$$ be a $$k$$-regular graph (each vertex have a degreee $$k$$). It is trivial to store the graph in $$O(\log n)$$ space or words such that $$j$$th neighbour of any vertex can be found in $$O(\log n)$$ time. Assume that neighbours of each vertex are ordered.

Note that $$k=O(\log n)$$

Is there an representation of graph $$G$$ that takes $$o(nk)$$ space in words such that query can be solved in $$O(1)$$

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## Show that,with the array representation for sorting an n-element heap, the leaves are the nodes indexed by n⌊n/2⌋+1,⌊n/2⌋+2,…,n

The Question of the CLRS $$6.1-7$$ exercise reads as:

Show that, with the array representation for sorting an n-element heap, the leaves are the nodes indexed by $$\lfloor n / 2 \rfloor + 1, \lfloor n / 2 \rfloor + 2, \ldots, n⌊n/2⌋+1,⌊n/2⌋+2,…,n$$.

I looked for the solution here: https://walkccc.github.io/CLRS/Chap06/6.1/

The solution was provided like this:

Let’s take the left child of the node indexed by $$\lfloor n / 2 \rfloor + 1.$$

\begin{aligned} \text{LEFT}(\lfloor n / 2 \rfloor + 1) & = 2(\lfloor n / 2 \rfloor + 1) \ & > 2(n / 2 – 1) + 2 \ & = n – 2 + 2 \ & = n. \end{aligned}

I can’t understand this statement: $$LEFT(⌊𝑛/2⌋+1) > 2(𝑛/2−1)+2$$

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## Short Version:

• Let all lower latin variables mean strings, except $$n, m, l, r$$, which are indices and index bounds.

• Let $$ab$$ mean usual string concatenation of $$a$$ to $$b$$.

• Let $$t \leqslant s \iff \exists$$ strings $$u,v$$ such that $$utv = s$$.

• Let $$\Sigma(s) = \{ a \leqslant s : |a| = 1\}$$.

• Let $$V(n) = \{X_1, \dots, X_n\}$$ be disjoint from $$\Sigma(s)$$ always.

• Let $$G(s, n) = \text{free-group}(V(n)\cup \Sigma(s))$$

• Let $$\text{pruned}(s, n) = \{$$ grammars $$g$$ for $$s$$ on $$\leq n$$ variables such that $$\forall X \in V(g)$$ we have $$X \leqslant g^k(S)$$ for some $$k \geq 0$$.

• Define a grammar $$g$$ on $$n$$ variables for $$s$$ to be a homomorphism of free groups: $$\alpha : G(s, n) \to G(s, n – 1)$$, such that $$\alpha$$ is some mapping $$V(n) \to G(s, n-1)$$ extended homomorphically, and identity on $$\Sigma(s)$$, and such that $$\alpha^k(S) = s$$ for all $$k \geq n_g$$.

• Let $$\text{SGP}(s, n) = \{\overline{x} \in G(s, n)^n :$$ if $$\overline{x}\cdot \overline{g} = \overline{g’}$$ then $$g’$$ can be pruned such that an equivalent grammar resides in $$\text{pruned}(s, n)$$ whenever $$g$$ does $$\}$$.

• From now on, identify $$G(s, n-1)$$ with the first $$n-1$$ components of $$G(s, n)$$.

• Question is at the bottom of post.

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## Long Version:

A grammar $$g$$ of a string $$s$$ over $$\Sigma$$ is a string homomorphism from $$(\Sigma \cup V)^* \to (\Sigma \cup V\setminus \{S\})^*$$ such that $$g^n(S) = s$$ for all sufficiently large $$n$$, where $$V$$ is a finite set of variable symbols, and $$S \in V$$ is the designated start symbol which is defined to be present for any string $$s$$. A pruned grammar is one such that for all $$X \in V(g),$$ the letter $$X$$ appears as a substring to some iterate of $$g$$ evaluated at $$S$$: $$X \leqslant g^k(S)$$ for some $$k \geq 0$$. A pruned grammar just gets rid of unused rules.

Define the size of a grammar $$g$$ of $$s$$ to be the sum of the lengths of the RHS of rules defininig $$g$$. Recall from context-free languages that a grammar can also be defined as a list of rules of the form $$X \to x$$. Formally we can express this as $$|g| := \sum_{X \in V(g)} |g(X)|$$.

In order to algebratize the problem, I didn’t find it beneficial to study string homomorphisms that are pruned grammars (the semigroup formed is $$(g \circ g’ = g’, \forall g,g’ \in \text{ the semigroup})$$. This is because composing two grammars like functions causes the first in the composition to hide the variables of the second, such that when the result is pruned you get the first operand as the result.

Instead, we’ll study transformations of the rules of grammars with $$n$$ rules. Consider the direct product $$G^n$$, where $$G$$ is the free group on $$\Sigma(g) = V(g) \cup \Sigma(s)$$, where the union is made disjoint by an unlimited supply of new variable symbols. Clearly if we assign a variable index to factor of the product, then we have an encoding of all posible grammars on $$n$$ variables, a lot them being redundant and “very similar” to other grammars.

None the less, the smallest grammar problem should be statable as:

Find a grammar $$g$$ of $$s$$ such that $$|g|$$ is minimized, or equivalently,

Find a vector $$\overline{g} \in G^n, n\geq 1$$ such that $$|g|$$ is minimized and $$g$$ is a grammar for $$s$$.

Note the $$\geq n$$ part. It implies that any naive algorithm would have to check all $$n = 1…|C(s)|$$ where $$|C(s)| = O(|s|^2)$$ and $$C(s)$$ is the set of substrings of $$s$$. I.e. in the worst case assume you have to variablize approximately all substrings. This is not the case in practice or in theory, but it shouldn’t matter for this analysis.

Now we have to be careful about how we define $$|\cdot| : G \to \Bbb{Z}$$ on a group, because it can literally be always equal to $$1$$ if not done right. Since $$G$$ is generated by $$\Sigma(g)$$, we say that $$|x| =$$ the length of a minimial run of letters in $$x_i \in \Sigma(g) \cup \Sigma(g)^{-1}$$. I.e. if $$x_1 \cdots, x_r = x$$. And we cannot do the same for $$r’ \lt r$$, then $$|x| := r$$.

Since the alphabet $$\Sigma(g)$$ depends on the grammar, we have to introduce the alphabet that doesn’t change over $$\overline{g} \in G^n$$. That, we’ll just call $$\Sigma_n = \{X_1, \dots, X_n\}$$.

Now to define $$|\overline{x}|$$ for $$\overline{x} \in G$$ we need to take out rules that are easily seen to be not required or not helpful in making a grammar the smallest it can be. For instance if $$X_1 \to a \in \Sigma(s)$$, then $$|a| = 1$$ and so any grammar would actually be larger than the equivalent grammar without it. Thus whenever $$t \in \Sigma_n \cup \Sigma$$ we define $$|t|_n = 0$$. To get to the equivalent non-redundantly styled grammar, you just substitute in $$a$$ for every occurence of $$X_1$$ found on the RHS of other rules.

Define a grammar symmetry from $$\overline{g}$$ to be any element $$\sigma \in G^n$$ such that when $$\sigma$$ is applied componentwise on the left (or right) of $$\overline{g}$$, written $$\overline{h} = \sigma \overline{g}$$ we have that $$h^k(S) = s$$ for sufficiently large $$k$$ is a preserved property.

We have $$G^n$$ acting on $$G^n$$ itself and a subset $$Y := \text{pruned-grammars}(n, s) = \{ \overline{g} : g$$ is a pruned grammar for $$s\}$$. The subset $$H = \text{Stab}_{G^n}(Y)$$ i.e. all group elements that stabilize pruned grammars for $$s$$ together with their componentwise inverses, is a subgroup of $$G^n$$. Proof. $$iY =Y, hY = Y \implies Y = h^{-1} Y \implies ih^{-1}Y = Y$$.

Let $$X := \text{smallest-grammars}(n, s)$$. Clearly $$X \subset Y$$ since you can’t have extra junk in a smallest grammar of course. Note: elements of $$X$$ are precisely the smallest grammars one can achieve in $$n$$ or less variables. The reason you can represent the lower variable counts in there as well, is that a component of $$\overline{g}$$ can simply map to $$1 \in \Sigma^*$$ and it has length $$0$$.

Lemma 0. $$H \times Y \to Y$$ acts transitively.

Let $$g, h$$ be two pruned grammars of $$s$$, each with $$n$$ rules, or $$g,h \in Y$$. Consider the component of lowest index $$i$$ such that $$g_i \neq h_i$$. Since we’re in a group, $$x g_i = h_i$$ has a solution for all $$g_i, h_i \in \text{Stab}_{G^n}(Y)$$. Similarly we have that $$g_i = x^{-1} h_i$$. Do the same for each component, and call $$x_j$$ the left multiplier to get from $$g_j$$ to $$h_j$$. Then $$\overline{x} = (x_1, \dots, x_n)$$ is such that $$\overline{x}\overline{g} = \overline{h}$$.

This is flawed, since $$\text{Stab}_{G^n}(Y)$$ may not be writeable as a direct product.

Lemma -1. $$\text{Stab}_{G^n}(Y)$$ is equal to a direct product of subgroups of $$G^n$$. Let $$K_i = \{ (1,1, \dots, x_i, \dots, 1, 1) : x_i$$, is in the $$i$$th position and $$(c_1, \dots, c_{i-1}, x_i, \dots, c_n) \in \text{Stab}_{G^n}(Y)$$ for some $$c_j \in G\}$$. Then $$K_i$$ forms a group under componentwise multiplicaiton.

Proof. We only need to show this for the $$i$$th component since the rest are all $$1$$. Without loss of generality we only need to show that $$H_i = \{ x \in G : (c_1, \dots, c_{i-1}, x, \dots, c_n) \in \text{Stab}_{G^n}(Y)$$ for some $$c_j \in G \}$$ is a subgroup of $$G$$. Since $$\text{Stab}_{G^n}(Y)$$ is in particular a subgroup of $$G^n$$ any multiplication of its elements must be carried out componentwise, by definitino of direct product. Let $$x, y\in H_i$$. Then clearly $$(c_1 d_1, \dots, c_{i-1}d_{i-1}, xy^{-1}, \dots, c_n d_n) \in \text{Stab}_{G^n}(Y)$$, where similarly we can prove, using component-wiseness that $$y^{-1} \in H_i$$. $$\blacksquare$$

That proves that $$\text{Stab}_{G^n}(Y)$$ transitively acts on $$Y$$. Define $$H’ := \text{Stab}_{H}(X) = \{ \overline{x} \in H : \overline{x} X = X \}$$ similarly, and we have a proof that $$H’ \times X \to X$$ is a transitive group action.

Thus if we are given a grammar $$\overline{y} \in Y$$ for $$s$$, say it might be approximately small, but not the smallest. Then there is a group element in $$H = \text{Stab}_{G^n}(Y)$$ that will take $$\overline{y}$$ to $$\overline{g}$$, a smallest grammar.

Thus the question can be phrased, compute $$\text{SGP}(s,n) = \text{argmin}_{\overline{x} \in H}\{|\overline{x} \cdot \overline{y}|\} \cdot \overline{y} =$$ the set of smallest grammars of $$s$$ on up to $$n$$ variables, when given a pruned grammar $$y$$ for $$s$$. You can go ahead and take $$y = \{ S \to s \}$$, with $$1$$‘s in the other components of $$\overline{y}$$.

## Questions.

1. For any smallest grammar $$g$$ does there always exist two other smallest grammars $$h_1, h_2$$ such that $$\overline{h_1}\overline{h_2} = \overline{g}$$ in the direct product $$G^n$$? In other words is there a recursive decomposition of the problem for each problem instance.

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## Find the binary representation of N

John was not happy with the rating he got for the annual year 2017-18. He asked his boss Jim about ratings. Jim asked him to solve a puzzle and it is giving ‘N’, find it in the binary representation of ‘N’, the number has ‘1’ followed by all zeroes. Hint : Use log

## How to transform an arbitrary graph into a fixed vector representation?

Actuality I work in computer vision, specifically on a problem known as “scene graph modeling.” This problem aims to convert an image $$I$$ in a graph $$G=(V,E)$$ where the nodes $$V$$ represent the objects (and the features) in scene and the edges $$E$$ the relationships between objects. An interesting paper on this topic is Graph R-CNN for Scene Graph Generation (Note that unlike of only to detect the objects in an image, the scene graph aims to capture the contextual information of image). A graph is a mathematics structure rich in information, and it would be very interesting to integrate graphs in a machine learn approach. In order to achieve this task is necessary to transform a graph in a vector representation. Some works that intend solve this problem are the following:

• SEMI-SUPERVISED CLASSIFICATION WITH GRAPH CONVOLUTIONAL NETWORKS: The problem with this algorithm is that assumes a fix number of nodes. After training, this algorithm take a graph $$G=(V,E)$$ as input  (whit $$N$$ nodes, that is, $$|V|=N$$) and outputs a fixed vector representation.
• graph2vec: Learning Distributed Representations of Graphs: This algorithm is flexible due to permit build a vector representation from a graph $$G$$ without restrict the number of nodes. However, it needs to know the whole space graph. That is, given a set $$G=\{g_{1},g_{2},\dots,g_{i},\dots,g_{m}\}$$, where $$g_{i}$$ is the i-th graph, this algorithm builds a vectorial representation $$V=\{v_{1},v_{2},\dots,v_{i},\dots,v_{m}\}$$, where $$v_{i}$$ is the i-th vector associated with the graph $$g_{i}$$. This algorithm is originally proposed to text analysis, where the features in nodes are of low dimension, I do not know if it can work using high dimension features in nodes.

I would like to know if there is another simple algorithm that allows me to convert any graph into a fixed vector representation.

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## Representation of the concatenation at the type level

I would like to learn more about concatenative programming through the creation of a small simple language, based on the stack and following the concatenative paradigm.

Unfortunately, I haven’t found many resources concerning concatenative languages and their implementation, so excuse me in advance for my possible naivety.

I therefore defined my language as a simple sequence of concatenation of functions, represented in the AST as a list:

data Operation     = Concat [Operation]     | Quotation Operation     | Var String     | Lit Literal     | LitOp LiteralOperation  data Literal     = Int Int     | Float Float  data LiteralOperation     = Add | Sub | Mul | Div 

The following program, 4 2 swap dup * + (corresponding to 2 * 2 + 4) once parsed, will give the following AST:

Concat [Lit (Int 4), Lit (Int 2), Var "swap", Var "dup", LitOp Mul, LitOp Add] 

Now I have to infer and check the types.

I wrote this type system:

data Type     = TBasic BasicType   -- 'Int' or 'Float'     | TVar String        -- Variable type     | TQuoteE String     -- Empty stack, noted 'A'     | TQuote String Type -- Non empty stack, noted 'A t'     | TConc Type Type    -- A type for the concatenation     | TFun Type Type     -- The type of functions 

That’s where my question comes in, because I don’t know what type to infer from that expression. The resulting type is obvious, it is Int, but I don’t know how to actually entirely check this program at the type level.

At the beginning, as you can see above, I had thought of a TConc type that represents concatenation in the same way as the TFun type represents a function, because in the end the concatenation sequence forms an unique function.

Another option, which I have not yet explored, would be to apply the function composition inference rule to each element of this expression sequence. I don’t know how it would work with the stack-based.

The question is so: how do we do it? Which algorithm to use, and which approach at the type level should be preferred?

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