Questions tagged [number-theory]
A mathematical puzzle whose solution is heavily based on the arithmetic properties of the integers. General number theory questions are off-topic but can be asked on Mathematics Stack Exchange.
257
questions
-2
votes
1answer
105 views
How many pigeons are in the flock? [closed]
A crow reaches a flock of pigeons. the crow asks the pigeons' leader: "How many of you are there?"
The pigeon replies: "We and we and a half of we and a fourth of we and you equal 100.&...
9
votes
1answer
354 views
The Lucky Number
Lucky numbers are 4 digit numbers that have the following property: they are equal to the sum of the fourth power of their digits. Therefore, they can be expressed as follows:
$$1000a+100b+10c+d = a^4+...
8
votes
1answer
330 views
The beginning of a factorial
Is there a positive integer n such that the decimal representation of n! starts with 123456789?
5
votes
2answers
329 views
Seventeen positive integers
Find 17 positive integers such that no four of them have, pairwise, a common divisor greater than 1, but, likewise, no four of them are, pairwise, relatively prime.
Do so in such a way that the ...
0
votes
1answer
135 views
On certain triplets of consecutive integers [closed]
While completelely factorizing integers, my student Luciana noticed that the canonical prime factorization of the three consecutive numbers 81=3^4, 82=2x41, and 83=83, use numbers which are all ...
7
votes
3answers
206 views
Perfect power nim
Let $m,n$ be positive integers. Ann and Ben has $m$ stones, and each of them takes exactly the perfect power of $n$ stones ($n^k$, where $k$ is a nonnegative integer) in order, starting from Ann. Who ...
3
votes
1answer
125 views
Fill in numbers on the cube … again!
You are given a cube. You are told to fill in each face randomly with some of the numbers $4, 5, 6, ..., 11$, with no repetition. What is the probability that for each two faces that are connected by ...
3
votes
1answer
118 views
Inheriting my father's “great” investment
My father, you see, was frankly quite gullible. Many years ago he fell for one of these "investment" scams. The scam promised to exactly double your investment every month, and excited by ...
4
votes
1answer
158 views
Fill in numbers on the cube!
You are given a cube. You are told to fill in each vertex with the numbers $4,5,6,...,11$, with no repetition. What is the probability that for each two vertices that are connected by a common edge, ...
5
votes
3answers
856 views
4k reputation special: “I hate square numbers!”
There is a large prison, with exactly 4000 prisoners. The warden noticed that there were too many prisoners, so they lined up all the prisoners, and repeated the following procedure until less than ...
10
votes
5answers
760 views
Length of an integer is part of its digits [closed]
How many positive integers less than 1,000,000,000 contain their length as part of their digit string?
Example: 123466 has a length of 6 and 6 is one of its digits. Hence this number needs to be ...
7
votes
3answers
344 views
The 15 Pebbles Game
This is a game for 2 players - Each player uses a different coloured marker or pencil, there are 15 pebbles in total.
Players take turns to colour 1, 2 or 3 pebbles (player chooses how many). When all ...
3
votes
2answers
227 views
Cannonballs packing
A pile of cannonballs stacked like a pyramid has a rectangular base. Each layer has a length and a width in terms of cannonballs that are each one less than those of the layer that is directly below. ...
13
votes
8answers
2k views
n(n+1) as a multiple of 100
Here's a puzzle I came up with while walking today:
For how many natural numbers $n$ is the number $n(n+1)$ a multiple of $100$?
This is true for infinitely many $n$, so "how many" means something ...
1
vote
2answers
122 views
Find a factor #1
The series of Find a factor puzzle is started by Culver Kwan, and asks the solver to identify a factor of a certain large number within a certain range using some mathematical identities. This should ...
9
votes
0answers
250 views
Dead By Daylight
This puzzle is a reference to a game, Dead By Daylight. Though you don't need a knowledge about the game. Consequently, there is no "video-games" tag.
Story
The Entity wants to have the ...
4
votes
1answer
168 views
What's the graph relation? #3
What's the relation that joins the nodes?
Previous
What's the graph relation? #1
What's the graph relation? #2
6
votes
2answers
382 views
Primes in a Line
Place the first 20 primes (2 to 71) in a line so that the sum or difference (or both) of any two primes that find themselves next to each other is always a perfect square.
For which other values of N ...
6
votes
1answer
153 views
Follow the path of relation through the grid #3
There is a relation between rectilinear-adjacent squares such that there is a unique rectilinear path from the top-left corner of the grid down to the bottom-right corner of the grid. Each square can ...
7
votes
1answer
175 views
Follow the path of relation through the grid #2
There is a relation between rectilinear-adjacent squares such that there is a unique rectilinear path from the top-left corner of the grid down to the bottom-right corner of the grid. Each square can ...
10
votes
2answers
272 views
Add a divisor! A game
Let $k$ be a positive integer. Amy and Ben are playing a game, with the number $1$ written on the whiteboard initially. Amy and Ben do the following in order, starting with Amy:
Suppose the number on ...
11
votes
1answer
300 views
Floor floor floor inside another floor
Inspired from this question
$$
\aleph(x,n)=\lfloor x\lfloor x\lfloor x...\rfloor\rfloor\rfloor\
$$
where $\aleph$ is the inner floor function with $n$ times for $x$. For example;
$$
\aleph(x,3)=\...
7
votes
3answers
189 views
The ceiling function and powers of 2
How many integers $1\le x\le2048$ such that
$$\Big\lceil \frac x{2^n}\Big\rceil$$ is not a multiple of five for all nonnegative integers $n$?
This problem is a 2020 contest problem which has finished....
0
votes
0answers
37 views
Brute force a keypad with minimal keystrokes [duplicate]
Senario
Say you have a keypad whose password is some two digit code which you do not know, say 34.
Entering digits in succession on this keypad eg. ...
45
votes
4answers
6k views
xāxāxāxāāā = 2020
Solve for $x$:
$$
x \left\lfloor x \left\lfloor x \left\lfloor x \right\rfloor \right\rfloor \right\rfloor = 2020.
$$
The floor function $\left\lfloor t \right\rfloor$ has the usual āgreatest integer ...
5
votes
2answers
386 views
Consecutive numbers which use all digits a different number of times
Are there arbitrarily long sets of consecutive numbers such that when writing the set down, every single digit (0 to 9) is used a different number of times?
4
votes
3answers
185 views
The death prism
One day, you are caught by a evil wizard. He presents you with a prism, and says, "You can ask me to turn this prism to any $n$-angled right prism. Then you shall fill in $1$ to $3n$ with no ...
6
votes
1answer
205 views
What's the graph relation? #2
What's the relation that joins the nodes? Open the image in a new tab if you'd like to see the diagram with better resolution.
Previous
What's the graph relation? #1
Hint 1
4
votes
1answer
97 views
6
votes
4answers
1k views
What is larger than largest, your intuitions are tarnished. (What am I?)
You might call me a number,
but that would be a blunder.
In class you may have been told otherwise,
but I tell you now that those were all lies.
I'm in the deck of the cards,
as big as them come.
...
8
votes
0answers
305 views
I'm a number with a special product, so name me when you think you've got it
In the blocks that come before,
their special product tells us more.
To guess my scheme you'll need calculation,
but only little tests of recreation.
Each block contains atoms strong as Thor,
...
10
votes
2answers
986 views
First digit of 2020!
This is a follow up to First digit of 3^2020
Can you find the first digit of 2020! (factorial) without a computer?
33
votes
3answers
5k views
First digit of 3^2020
Inspired by
The last digit for 3^(2019)
Can you find the first digit of $3^{2020}$ without a computer?
6
votes
2answers
161 views
The Balls of Death
You have nightmares about a pool ball with the number 1 on it, and an empty box. Why?
Immortality
Imagine, if you will, we are 5000 years into the future. Medicine has evolved and we now are immortal -...
2
votes
2answers
728 views
20
votes
4answers
2k views
What's in my pocket?
Well, I can tell you Johnny has memory cards in his pocket.
Back Story
My brother, Johnny, is a tech nerd. He loves gadgets of all kinds. As a matter of fact, you can be sure at any one given time, he ...
9
votes
3answers
442 views
Number Guessing (Part 1)
I thought up two positive integers with product less than $500$. I told their product to Penny, and their sum to Sandy, and told both of them the constraints and they are both perfect logicians.
They ...
5
votes
1answer
8k views
How to programmatically solve math puzzle
I have this puzzle and I want to solve by code. I wrote simple code trying to brute force but it fail in one condition
https://dotnetfiddle.net/cJyu3w
Anyone know of a link or source how to solve ...
19
votes
1answer
1k views
Four mathematicians and their ages
Four mathematicians, none yet a centenarian, meet for coffee. The graph-theorist among them noticed that the common divisor graph of their ages (that is, the graph whose vertices are their ages, two ...
4
votes
1answer
148 views
Playing Collatz with graphs
a) Given a set of positive integers, its common divisor graph (CD-graph) is the graph whose vertices are the integers, two of which are joined by an edge if (and only if) they have a common divisor ...
11
votes
5answers
2k views
Nice no-computers way to find limerick primes?
A limerick number is a 5-digit number whose digits are in the form of a limerick rhyme scheme: $aabba$. How many limerick primes (limerick numbers which are also prime) are there?
The answer is
...
12
votes
4answers
1k views
Professor Halfbrain and the number cycles
Yesterday afternoon I met professor Halfbrain at an art gallery.
The professor looked tired and exhausted.
He told me that he had spent many working days and many sleepless nights
with lengthy ...
-6
votes
1answer
212 views
Puzzling set of numbers [closed]
Let's have the following numbers $23, 40, 42, 44, \sqrt{43},\ 128i, 130, \sqrt{172}\ $. What is the relationship between these numbers, taken four at a time?
There are only two combinations when you ...
15
votes
6answers
645 views
A special triple of factors
Using each of the digits 1,2,3,4,5,6,7,8,9 exactly once, create three 3-digit numbers such that their product is a maximum.
5
votes
1answer
173 views
Generating special numbers [closed]
Here are three numbers that are related to one another: 9841, 8591, 4800
How are these numbers related and how are they generated?
11
votes
4answers
911 views
Making the whole set into primes
Let's say you start with a set of sequential integers starting from 2, so: $ 2, 3, 4, 5, \dots, N $ for some $ N > 2. $
The goal is to use identical basic arithmetic operations ($ +, -, \times, \...
6
votes
3answers
209 views
Find the value of $\bigstar$: Puzzle 10 - Uncertainty
This puzzle replaces all numbers with other symbols.
Your job, as the title suggests, is to find what value fits in the place of $\bigstar$. To get the basic idea, I recommend you solve Puzzle 1 first....
8
votes
3answers
650 views
Discover the six-character password!
You are given several pieces of paper which are as follows:
(Unfortunately, a textual rendering is very difficult with this puzzle, so if someone can offer a suggestion on how to do it, that would be ...
0
votes
0answers
81 views
Hitting a car with a bullet [duplicate]
1) There is a road on which a car starts with an integral speed towards the left or the right, starting from an integral point (take the road to be the x-axis)
2) The speed and the point from which ...
7
votes
2answers
439 views
Smallest prime number which when spelt out contains the letters P, R, I, M, E
So inspired by recent slew of questions based on prime numbers.
What is the smallest prime number when written out (Using the Western numbering system and English) would you encounter the letters P, ...
