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### Martin Roberts

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Shared publicly -**A Curious Property of 82000**

The number

**82000**in base 10 is equal to 10100000001010000 in base 2, 11011111001 in base 3, 110001100 in base 4, and 10111000 in base 5. It is the smallest integer bigger than 1 whose expressions in bases 2, 3, 4, and 5 all consist entirely of zeros and ones.

What is remarkable about this property is how much the situation changes if we alter the question slightly. The smallest number bigger than 1 whose base 2, 3, and 4 representations consist of zeros and ones is 4. If we ask the same question for bases up to 3, the answer is 3, and for bases up to 2, the answer is 2. The question does not make sense for base 1, which is what leads to the sequence in the picture: [undefined], 2, 3, 4, 82000.

The graphic comes from a blog post by

**Thomas Oléron Evans**. Most of the post discusses the intriguing problem of finding the next term in this sequence, and whether the next term even exists. In other words, does there exist an integer greater than 1 whose representations in bases 2, 3, 4, 5, and 6 all consist entirely of zeros and ones?

The number 82000 does not satisfy these conditions, because the representation of this number in base 6 is 1431344. This means that the next number in the sequence, if it exists, must be some number bigger than 82000 whose representations in bases 2, 3, 4, and 5 all consist entirely of zeros and ones. Unfortunately, even these weaker conditions are very difficult to satisfy. An exhaustive search has been carried out up to 3125 digits in base 5 and no solution exists in this range.

The upshot of this is that, if the next term in the sequence exists, it must have more than 2184 digits in base 10. (The 2184 comes from multiplying 3125 by the base 10 logarithm of 5.) However, there is also no known proof that the next term in the sequence does not exist.

**Relevant links**

Thomas Oléron Evans's blog post has much more discussion of this problem, at http://www.mathistopheles.co.uk/maths/covering-all-the-bases/solution-covering-all-the-bases/

Details of the exhaustive search can be found in the notes to the sequence http://oeis.org/A146025 in the

*On-Line Encyclopedia of Integer Sequences*.

There is a nice online number base converter tool at http://www.cleavebooks.co.uk/scol/calnumba.htm

#mathematics #sciencesunday

### Martin Roberts

Shared publicly -**What Happens When You Drop A Magnet Through A Copper Tube?**

The magnet induced a current in the copper pipe, which in turn produced a magnetic field. The direction of this current then opposed the change in the magnet’s field, resulting in the magnet being repelled and thus falling more slowly. Neat.

src: http://goo.gl/oME6U4 #ScienceIsAwesome #Science #ScienceSunday

### Martin Roberts

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Shared publicly -**Robot Master Chef Cooks 2,000 Recipes, Cleans Up, Does the Dishes**

*Would you buy it?*

More at: http://www.industrytap.com/robot-master-chef-cooks-2000-recipes-cleans-dishes/28765

### Martin Roberts

Shared publicly -I have always loved how something as simple as a pentagon can lead to such non-trivial mathematical connections such as the golden ratio.

A basic intermediate step provides the radius for both the inscribed and circumscribed #circle , which are in themselves interesting.

Where index n is used, the equations are valid for any regular polygon. For the pentagon in particular, we take another step to find that the d/s ratio is the golden ratio. #goldenratio

For other regular polygons, this ratio can be calculated with 2*cos(π/n), leading to the same results that are found with just #trigonometry , which is shown between these two steps.

### Martin Roberts

Shared publicly -Having completed my PhD in nuclear physics, I spent some time doing computational physics in medical research, and then worked for a web startup company at the height of the web 2.0 craze. Combining these interests with my professional training as a high school math teacher, I later joined the millions of others and developed some educational and math apps for the iPhone and iPad.

I spend my days as an Assistant Director at Australia's National Statistical Agency managing the dissimenination of their statistical collections, and in true ninja style, by night I hack away as an covert Data Scientist attempting to devise new and improved statistical machine learning algorithms for snazzy stuff applications like Automated Essay Scoring.