Tagged Questions
0
votes
0answers
27 views
Reversible smoothing of a two dimensional function (or an image)
Smoothing of an image, or a two dimensional function is quite easy, there are many methods to achieve it, using average of near elements.
But how to make it reversible?
Maybe DCT (discrete cosine ...
0
votes
1answer
40 views
How to derive this formula about the bracket function?
Is there a direct way of proving that
$$ [nx] = [x] + [x+\frac{1}{2}] + [x+\frac{1}{3}] + \ldots + [x+ \frac{1}{n}]$$
for each real number $x$ and for each positive integer $n$?
My effort:
Let ...
0
votes
0answers
21 views
Max Function Notation [duplicate]
I've been asked whether the following is always, never or sometimes true:
$f(n) + g(n) = \theta(\max(f(n), g(n)))$
I understand the definition of theta notation, but I'm not sure how to read the ...
1
vote
0answers
15 views
Existence of a particular transformation
I've a set of data points $S = \{ x | x\in [0,1]\}$ (i.e. real values from the unit interval). In some cases I've big clusters in the data and I want to spread the values in between the unit interval ...
0
votes
3answers
25 views
Prove a function is surjective or injective
so I'm having trouble figuring out why this question is surjective / where $0$ comes from.
$f : \mathbb{N} \longrightarrow \mathbb{N}$ where $f(x) = x + 1$.
so given N begins from $0$ it goes:
...
0
votes
1answer
14 views
Classifying relations as functions
so I'm having a bit of trouble actually working out the relation here, I'm fine with determining whether its a function/partialfunction etc I just don't understand how to produce the relation and get ...
2
votes
2answers
65 views
Constructing a function similar to x^3 between [0,1]
I'm trying to construct a function $f$, in order to normalize a dataset(obviously where all the element come from $[0,1] \in \mathbb{R}$.
The big picture is that the envisioned $f: [0,1] \rightarrow ...
0
votes
2answers
61 views
Is $x^2+25x+4 \in \mathcal{O}(x^2)$? If yes how? If no why not? [closed]
Is $x^2+25x+4 \in \mathcal{O}(x^2)$ ? if yes how ?, if no why?
i know
x^2+25x+4≤25x^2+25x+25≤25x^2+25x^2+25x^2=75x2 for some x
what confuses me is
x^2+25x+4≤25x^3+25x+25≤25x^3+25x^3+25x^3=75x3 ...
0
votes
3answers
56 views
How can I calculate summations of modulus expressions?
I know that the following holds true: $$\sum_{k=1}^n k = {n (n + 1) \over 2} $$
Can modulus expressions be simplified in similar ways?
For example:
$$\sum_{k=1}^n (k\bmod x) = ?? $$
To be clear, ...
1
vote
1answer
36 views
Discrete Math - onto, 1-1 functions
Let $S = \{3,B\}$
Give an example of a function $f: S \times S \to S$ that is onto.
Give an example of a function $g: S \to S \times S$ that is 1-1.
Give an example of a function $h: ...
2
votes
4answers
40 views
Trying to understand how many functions there are from A to B.
I'm trying to understand why there are $B^A$ functions from $A$ to $B$. If $A=${$a,b$} and $B=${$1,2,3$} then the functions from $A$ to $B$ are $f(a)=1$, $f(b)=1$, $f(a)=2$, $f(b)=2$, $f(a)=3$, ...
0
votes
1answer
31 views
Prove that function f is injective, if function (g o f) is injective as well
I'm sort of stuck with this type of proof. Not quite sure how to go about it. I was wondering if someone could help me out to get started with it.
I guess that the two hypotheses I have are these:
...
1
vote
2answers
39 views
Trouble understanding Big O notation for a sum of n integers [duplicate]
This problem is an example in a Discrete Math textbook. How can big-O notation be used to estimate the sum of the first n positive integers?
Solution: Because each of the integers in the sum of the ...
0
votes
1answer
11 views
Function form understanding
I dont understand what is the function when im given this kind of form:
f= {<1,1>,<2,3>,<3,2>}.
I understand functions when they are given in lambda form.
For example how can I find the ...
0
votes
1answer
38 views
Defining a bijective function from $2\mathbb{N}$ to $3\mathbb{Z}-1$?
$2\mathbb{N}=\{2n:n\in\mathbb{N}\}$ and $3\mathbb{Z}-1=\{3n-1:n\in\mathbb{Z}\}$
Work: So far, my plan is to first define a bijective function from $2\mathbb{N}$ to $\mathbb{N}$ and then define ...
0
votes
3answers
52 views
Proving that function $f:[0,\infty)\rightarrow [0,\infty)$ defined by $f(x)=\frac{x^2}{1-x}$ is bijective.
I am having a bit of trouble with the algebra for proving that the function is injective.
Basically I set $f(a)=f(b)$ for $a,b\in[0,\infty)$ and $a,b\neq 1$.
...
0
votes
2answers
27 views
Bijection of a function.
Define the function f: $(2,\infty) -> (-\infty,-1)$ by $f(x)= \frac{-x}{x-2}$. Show that f is bijective.
I know i need to prove both injective and surjective, and I was able to solve the equation ...
1
vote
2answers
82 views
How to prove a function from $\mathbb N\times \mathbb N$ to $\mathbb N$ is bijective. [duplicate]
I am having trouble with this problem:
$f\colon \mathbb N\times \mathbb N \rightarrow \mathbb N$ is defined by
$f(i,j)=\dfrac{(i+j-1)(i+j-2)}{2}+i$.
How do you prove that $f$ is a bijection from ...
1
vote
2answers
55 views
Proving that a function from $N\times N$ to $N$ is bijective.
I am stuck on this problem:
Define $f: N\times N \rightarrow N$ by
$f(i,j)=\frac{(i+j-1)(i+j-2)}{2}+i$.
How do you prove that $f$ is a bijection thus $N\times N$ and $N$ are numerically ...
0
votes
0answers
41 views
Proving that the set of irrational numbers is uncountable [duplicate]
Work: Assume that the set of irrational numbers is countable. Since $Q$ is infinite, it is therefore denumerable. Therefore, there exists a bijective function $f: N \rightarrow Q$. From here I am ...
1
vote
1answer
38 views
Proving that $f: N\times N \rightarrow N$ is surjective [duplicate]
I am having trouble proving that the function $$f: N\times N \rightarrow N, \ \ f(i,j)=2^{i-1}(2j-1)$$ is surjective.
Work: I know that using the theorem in which $n$ is the product of prime numbers ...
0
votes
2answers
62 views
Proving that $f$ is a bijection from $N$x$N$ to $N$.
I am having trouble with the following problem:
$f: N\times N\rightarrow N$ and $f(i,j)=2^{i-1}(2j-1)$. Prove that $f$ is a bijection thus $N\times N$ and $N$ are numerically equivalent.
Work: I ...
2
votes
5answers
670 views
Explanation of recursive function
Given is a function $f(n)$ with:
$f(0) = 0$
$f(1) = 1$
$f(n) = 3f(n-1) + 2f(n-2)$ $\forall n≥2$
I was wondering if there's also a non-recursive way to describe the same function.
WolframAlpha tells ...
2
votes
3answers
45 views
Why is the inverse of this function not a function?
Why does $F^{-1}$ need to be defined on all of $Y$? I can have this function: $g(x)=x,\quad x\ne 3$ and even though it is not defined for all $x$ in its domain, it is still a function, right?
0
votes
1answer
42 views
How can a function not be one to one and be a function?
My understanding of the definition of a function
Given any x, there is only one y that can be paired with x
My understanding of a 1 to 1 function
Given any y, there is only one x that can be paired ...
1
vote
1answer
27 views
Help with composite identity functions in discrete mathematics
I am having trouble with the following problem:
For nonempty sets $A$ and $B$ and functions $f : A \rightarrow
B$ and $g: B \rightarrow A$ suppose that $g \circ f = i_A$, the
identity function on ...
0
votes
1answer
8 views
Does each element in domain need result for onto functions?
For onto functions, do all the elements in the domain have to give a result from the range? I know that for one-to-one, every single $x$ must give a result, and one that is a unique $y$. For onto ...
1
vote
1answer
42 views
Help with identity functions in discrete mathematics
I have trouble with trying to solve the following problem:
For nonempty sets $A$ and $B$ and functions $f:A\rightarrow B$ and $g:B\rightarrow A$ suppose that $g\circ f=i_A$, the identity function on ...
-1
votes
3answers
57 views
Proving functions are injective and surjective
I am having trouble with the following problem:
For nonempty sets $A$ and $B$ and functions $f:A \rightarrow B$ and $g:B \rightarrow A$ suppose that $g\circ f=i_A$, the identity function of $A$. ...
0
votes
3answers
35 views
Injective and Surjective Function Examples
I am having trouble with this problem:
Give an example of a function $f:Z \rightarrow N$ that is
a. surjective but not injective
b. injective but not surjective
Work: I came up with examples such ...
0
votes
1answer
20 views
Proving Integer Modulo is Well-Defined
I have trouble figuring out this problem:
$h: Z_4 \rightarrow Z_6$ by $h([a])=[3a]$ for each $a\in Z$.
Prove that h is well-defined thus it is a function and that h is neither injective nor ...
0
votes
1answer
40 views
Confusion surrounding functions
Hey there Mathematics,
Slightly confused over some of the things in my quiz and was wondering if I could get an explanation:
I thought with the first question that it's just one-to-one from X to Y ...
1
vote
2answers
24 views
Help with Discrete Math Functions and Bijections
I have trouble with the following problem:
Prove that the function $f(x)=x^2-2x+3$, with domain $x\in (-\infty, 0)$, is a bijection from $(-\infty, 0)$ to its range.
Work: I tried to first prove ...
0
votes
1answer
35 views
Help with Functions in Discrete Mathematics
I am having trouble solving this problem:
let $p$ be a positive prime number and let $f:Z_p -> Z_p$ be defined as $f([x])=[x^2]$. Show that $f$ is a function. Give examples of how it is not ...
0
votes
1answer
43 views
Equivalence Relation on R (real numbers)
Let R be the relation on R(real numbers) defined by:
For all x, y (that belong) to R(real numbers), x relates y <=> x-y (that belongs) to Z.
(a) Is R an equivalence relation? Prove your answer.
...
-1
votes
2answers
138 views
Give a Bijection that is in-between 2 intervals and use a formal proof to show that it is a bijection. [duplicate]
∀w,x,y,z ∈ R, w < x and y < z. Given that information, supply a bijection between the two intervals. (w,x) and (y,z) Then after you find the bijection, provide a formal proof that what you found ...
1
vote
1answer
59 views
Let A = {1,2,3,4} Let F be the set of all functions from A to A. (check the parts)
Let $\operatorname{S}$ be a relation on $F$ defined by: $\forall f, g \in F, f\,\operatorname{S}\,g \iff f(i) = g(i), \exists i \in A$.
(a) Recall that the identity function $I_A : A \mapsto A$ is ...
0
votes
1answer
31 views
How many functions with A having 9 elements and B having 7 elements have only 1 element mapped to 7?
So, the question I have is: How many onto functions [9] --> [7] have only one element mapped to 7?
This is asking how many functions with A having 9 elements and B having 7 elements have only 1 ...
0
votes
1answer
106 views
Discrete Math: Functions and Set Questions
1) Consider the function: $f: \mathbb{R} \to \mathbb{R}$ (Real to Real Number), where $f(x)=2+x^2$, what would be all of the preimages of $3$?
1) $11$
2) $11$, $-11$
3) $1$, $-1$
4) $1$
2) Let $D ...
1
vote
1answer
45 views
A one-to-one function from $\mathbb{Z}^+ \to (0,1)$?
I ran into a interesting homework question that asked me to find an example of a one-to-one function $f: \mathbb{Z}^+ \to (0,1)$. I'm thinking it should be some kind of linear function, or polynomial ...
1
vote
1answer
73 views
Let $S =\{1,2,3,4,5 \} $ For each give a brief explanation and simply answer to a number. (a) How many functions $f: S \longrightarrow S$ are there?
(b) How many one-to-one functions $f : S\longrightarrow S$ are there?
(c) How many functions $f : S \longrightarrow S$ are there so that $f o f(1) = 2$ ?
(d) How many onto functions $f: ...
0
votes
1answer
73 views
Prove that this function defined by f(x) is bijective.
Prove that the function $ f: \mathbb{R} - \{1\} \to \mathbb{R} - \{1\}$ defined by $ f(x) $ is bijective.
$$ f(x)=\left({x+1\over x-1}\right)^3 $$
I am taking my first Computer Science ...
0
votes
3answers
161 views
Proving a function is a one to one correspondence
I understand that to show a function is a one to one correspondence, you have to show that the function is both one to one and onto. Proving a function is one to one seems simple enough. However for ...
3
votes
1answer
45 views
Discrete math functions help?
I'm doing a review for my discrete math test on functions and I'm having troubles with a few questions. Can I get some guidance in how to do these questions so I can be more prepared for the test? ...
0
votes
3answers
19 views
Function Mappings
There are two “shift functions” mapping $\Bbb N$ into $\Bbb N$: $f(n)=n+1$ and $g(n)=\max\{0,n-1\}$ for $n\in\Bbb N$.
How to show that $(g\circ f)(n)=n$ for all $n$, but that $(f\circ g)(n)=n$ does ...
0
votes
3answers
41 views
Prove that a function is one to one without graphing
I know that you can prove a function is one to one by graphing it and using the horizontal line test. But in my notes it showed another way to prove a function is one to one but I am not sure if I am ...
1
vote
2answers
43 views
An example of a function whose domain is the set of positive integers and range is the set of integers?
I was browsing through one of my old pre-calc books, and I feel a bit ashamed to say I can't think of a simple answer. It intuitively feels impossible, as there are half as many points in the domain ...
0
votes
4answers
51 views
How many different functions $f: A \rightarrow B$ are there if $|A| = m$ and $|B| = n$
I'm not quite sure of what this question is asking. Can someone explain please
3
votes
0answers
60 views
Find generating function For sequences
Can anyone out here help?
The exercise says: "Find the generating function for each of the sequences below (the general term is given)"
Now, the question is how do you find one for those:
a) $U_n = ...
0
votes
1answer
16 views
One to one function behaviour
Like in pigeon hole principle , if one set of objects(S1) has more items than others set of objects(S2) and we try to fit that S1 in S2 ( that is mapping the values of S1 to S2 , we end up getting ...
