Taking a break

Taking a break

If you are following my blog regularly, you will know that I usually posts once a week on Saturday. However, nothing new has been posted the last couple of weeks. I just wanted to let you know that the site has not been abandoned or anything like that. However, I do have a lot of things going on right now and only 24 hours per day, and that meant I had to stop doing something. Posting on my blog was one of the things.

I will pick it up again someday, I know I will. I will also try to come up with a few different other posts and topics. I already have one idea which you will see in a few weeks I hope.

 

 

Posted by Kristian in Other, 4 comments
Project Euler 146: Investigating a Prime Pattern

Project Euler 146: Investigating a Prime Pattern

In Problem 146 of Project Euler we are working with primes again, and some quite big ones even. The problem reads

The smallest positive integer n for which the numbers n2+1, n2+3, n2+7, n2+9, n2+13, and n2+27 are consecutive primes is 10. The sum of all such integers n below one-million is 1242490.

What is the sum of all such integers n below 150 million?

At first I thought I could just make a sieve up to 150 million and then check if the numbers were contained in that. However, rereading the problem I realized I was completely wrong. So in a pure brute force solution we would need to check 150 million values of n and up to 13 numbers for each, since we both need to check that the given numbers are prime. But also that the odd numbers inbetween are not prime. So potentially we have to check 1950 million numbers for primality, which is a moderately expensive operation. Continue reading →

Posted by Kristian in Project Euler, 7 comments
Project Euler 145: How many reversible numbers are there below one-billion?

Project Euler 145: How many reversible numbers are there below one-billion?

In Problem 145 of Project Euler we move away from Geometry and over to number theory again, with a problem which reads

Some positive integers n have the property that the sum [ n + reverse(n) ] consists entirely of odd (decimal) digits. For instance, 36 + 63 = 99 and 409 + 904 = 1313. We will call such numbers reversible; so 36, 63, 409, and 904 are reversible. Leading zeroes are not allowed in either n or reverse(n).

There are 120 reversible numbers below one-thousand.

How many reversible numbers are there below one-billion (109)?

This one is insanely easy to write a brute force method and that is the first thing I did. However, as we shall see there is a more analytic approach to the problem as well. Continue reading →

Posted by Kristian in Project Euler, 5 comments
UVa Online Judge New Platform

UVa Online Judge New Platform

We at MathBlog have written about a couple of problems from the UVa Online Judge. The current UVa OJ platform is almost 7 years old, and so the UVa OJ team thinks it’s time for an upgrade. They intend to build the platform up from scratch, but to do that, they need funding. Therefore, they’ve set up a campaign, located here, where you can donate and help fund the project. Note that MathBlog is not affiliated with UVa Online Judge in any way.

There are 6 days left of the campaign, so we encourage you to let others know, and perhaps even donate yourself.

Again, here is the campaign, and here is the main UVa OJ website.

Posted by Bjarki Ágúst in UVa Online Judge, 1 comment
Project Euler 144: Investigating multiple reflections of a laser beam.

Project Euler 144: Investigating multiple reflections of a laser beam.

Problem 144 of Project Euler is once again a geometry problem, just like the previous. However, it is completely different. The problem reads

In laser physics, a “white cell” is a mirror system that acts as a delay line for the laser beam. The beam enters the cell, bounces around on the mirrors, and eventually works its way back out.

The specific white cell we will be considering is an ellipse with the equation 4x2 + y2 = 100

The section corresponding to -0.01  x  +0.01 at the top is missing, allowing the light to enter and exit through the hole.

The light beam in this problem starts at the point (0.0,10.1) just outside the white cell, and the beam first impacts the mirror at (1.4,-9.6).

Each time the laser beam hits the surface of the ellipse, it follows the usual law of reflection “angle of incidence equals angle of reflection.” That is, both the incident and reflected beams make the same angle with the normal line at the point of incidence.

In the figure on the left, the red line shows the first two points of contact between the laser beam and the wall of the white cell; the blue line shows the line tangent to the ellipse at the point of incidence of the first bounce.

The slope m of the tangent line at any point (x,y) of the given ellipse is: m = -4x/y

The normal line is perpendicular to this tangent line at the point of incidence.

The animation on the right shows the first 10 reflections of the beam.

How many times does the beam hit the internal surface of the white cell before exiting?

Continue reading →

Posted by Kristian in Project Euler, 12 comments
Project Euler 143: Investigating the Torricelli point of a triangle

Project Euler 143: Investigating the Torricelli point of a triangle

Problem 143 of Project Euler is a notorious problem. Notorious for having the fewest correct answers per time it has been released. If you sort by number of solvers, you will see a pretty good correlation between problem number and place on that list. However, this problem is moved quite a bit down that list. The problem reads

Let ABC be a triangle with all interior angles being less than 120 degrees. Let X be any point inside the triangle and let XA = p, XB = q, and XC = r.

Fermat challenged Torricelli to find the position of X such that p + q + r was minimised.

Torricelli was able to prove that if equilateral triangles AOB, BNC and AMC are constructed on each side of triangle ABC, the circumscribed circles of AOB, BNC, and AMC will intersect at a single point, T, inside the triangle. Moreover he proved that T, called the Torricelli/Fermat point, minimises p + q + r. Even more remarkable, it can be shown that when the sum is minimised, AN = BM = CO = p + q + r and that AN, BM and CO also intersect at T.

If the sum is minimised and a, b, c, p, q and r are all positive integers we shall call triangle ABC a Torricelli triangle. For example, a = 399, b = 455, c = 511 is an example of a Torricelli triangle, with p + q + r = 784.

Find the sum of all distinct values of p + q + r ≤ 120000 for Torricelli triangles.

Continue reading →

Posted by Kristian in Project Euler, 4 comments
Project Euler 141:Investigating progressive numbers, n, which are also square.

Project Euler 141:Investigating progressive numbers, n, which are also square.

Problem 141 of Project Euler proved to be just as difficult as the number of people who has actually solved it shows.  The problem reads

A positive integer, n, is divided by d and the quotient and remainder are q and r respectively. In addition dq, and r are consecutive positive integer terms in a geometric sequence, but not necessarily in that order.

For example, 58 divided by 6 has quotient 9 and remainder 4. It can also be seen that 4, 6, 9 are consecutive terms in a geometric sequence (common ratio 3/2).
We will call such numbers, n, progressive.

Some progressive numbers, such as 9 and 10404 = 1022, happen to also be perfect squares.
The sum of all progressive perfect squares below one hundred thousand is 124657.

Find the sum of all progressive perfect squares below one trillion (1012).

I ended up getting the right idea when I was working out. I guess some times it really does help to do something else. In this problem it comes down to some really basic properties and insights so lets start with those Continue reading →

Posted by Kristian in Project Euler, 22 comments
Project Euler 140: Modified Fibonacci golden nuggets

Project Euler 140: Modified Fibonacci golden nuggets

Problem 140 of Project Euler is very much a continuation of the Problem 137, as we can see from the problem description

Consider the infinite polynomial series AG(x) = xG1 + x2G2 + x3G3 + …, where Gk is the kth term of the second order recurrence relation Gk = Gk-1 + Gk-2, G1 = 1 and G2 = 4; that is, 1, 4, 5, 9, 14, 23, … .

For this problem we shall be concerned with values of x for which AG(x) is a positive integer.

The corresponding values of x for the first five natural numbers are shown below.

x AG(x)
(5-1)/4 1
2/5 2
(22-2)/6 3
(137-5)/14 4
1/2 5

We shall call AG(x) a golden nugget if x is rational, because they become increasingly rarer; for example, the 20th golden nugget is 211345365.

Find the sum of the first thirty golden nuggets.

In Problem 137 I mentioned in the end that the problem could be solved using a Diophantine equation. This is exactly the way I will go for this problem. Continue reading →

Posted by Kristian in Project Euler, 6 comments
Project Euler 139: Pythagorean tiles

Project Euler 139: Pythagorean tiles

In Project Euler There are loads of problems that end up with a number theoretic solution. Problem 139 is no exception to that.  The problem reads

Let (abc) represent the three sides of a right angle triangle with integral length sides. It is possible to place four such triangles together to form a square with length c.

For example, (3, 4, 5) triangles can be placed together to form a 5 by 5 square with a 1 by 1 hole in the middle and it can be seen that the 5 by 5 square can be tiled with twenty-five 1 by 1 squares.

However, if (5, 12, 13) triangles were used then the hole would measure 7 by 7 and these could not be used to tile the 13 by 13 square.

Given that the perimeter of the right triangle is less than one-hundred million, how many Pythagorean triangles would allow such a tiling to take place?

Continue reading →

Posted by Kristian in Project Euler, 6 comments
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