It appears that there was a club and the president decided that it would be nice to hold a dinner for all the members. In order not to give any one member prominence, the president felt that they should be seated at a round table. But at this stage he ran into some problems. It seems that the club was not all that amicable a little group. In fact each member only had a few friends within the club and positively detested all the rest. So the president thought it necessary to make sure that each member had a friend sitting on either side of him at the dinner. Unfortunately, try as he might, he could not come up with such an arrangement. In desperation he turned to a mathematician. Not long afterwards, the mathematician came back with the following reply. ‘It’s absolutely impossible! However, if one member of the club can be persuaded not to turn up, then everyone can be seated next to a friend.’ ‘Which member must I ask to stay away?’ the president queried. ‘It doesn’t matter,’ replied the mathematician. ‘Anyone will do.’

This problem, dubbed “Le Cercle Des Irascibles,” was posed by René Sousselier in Revue Française de Recherche Opérationelle in 1963. The remarkable solution was given the following year by J.C. Herz. In this figure, it’s possible to visit all 10 nodes while traveling on line segments alone, but there’s no way to close the loop and return to the starting node at the end of the trip (and thus to seat all the guests at a round table). But if we remove any node (and its associated segments), the task becomes possible. In the language of graph theory, the “Petersen graph” is the smallest hypohamiltonian graph — it has no Hamiltonian cycle, but deleting any vertex makes it Hamiltonian.

(Translation by D.A. Holton and J. Sheehan.)

Pick any point in the interior of an equilateral triangle and draw a perpendicular to each of the three sides. The sum of these perpendiculars is the height of the triangle.

That’s Viviani’s theorem. This visual proof is by CMG Lee:

1. Choose point P and draw the three perpendiculars.
2. Now draw three lines through P, each parallel to a side of the main triangle. This creates three small similar triangles.
3. Because these smaller triangles are equilateral, we can rotate each so that its altitude is vertical.
4. Because PGCH is a parallelogram, we can slide triangle PHE to the top, and now the heights of the three constituent triangles sum to that of triangle ABC.

The converse of the theorem is also true: If the sum of the perpendiculars from a point inside a triangle to its sides is independent of the point’s location, then the triangle is equilateral.

From Lee Sallows:

(Thanks, Lee!)

The so-called four-field approach in anthropology divides the discipline into four subfields: archaeology, linguistics, physical anthropology, and cultural anthropology.

Students call these “stones, tones, bones, and thrones.”

A centered hexagonal number is a number that can be represented by a hexagonal lattice with a dot in the center, like so:

Starting at the center, successive hexagons contain 1, 7, 19, and 37 dots. The sequence goes on forever.

The sum of the first n centered hexagonal numbers is n³, and there’s a pretty “proof without words” to show that this is so:

Instead of regarding each figure as a hexagon, think of it as a perspective view of a cube, looking down along a space diagonal. The first cube here contains a single dot. How many dots must we add to produce the next larger cube? Seven, and from our bird’s-eye perspective this pattern of 7 added dots matches the 7-dot hexagon shown above. The same thing happens when we advance to a 3×3×3 cube: This requires surrounding the 2×2×2 cube with 19 additional dots, and from our imagined vantage point these again take the form of a hexagonal lattice. In the last image our 3³ cube must accrete another 37 dots to become a 4³ cube … and the pattern continues.

Reader Derek Christie sent in this surprising curiosity after Wednesday’s post about Borromean rings:

Both the ring and the karabiner clip are attached to the cord and can’t be removed.

Now complicate matters by clipping the karabiner onto the ring:

Quite unexpectedly, the cord can now be just pulled away:

(Thanks, Derek.) (A related perplexity: The Prisoners’ Release Puzzle.)

“Rules for the direction of the mind,” from an unfinished treatise by René Descartes:

1. The aim of our studies must be the direction of our mind so that it may form solid and true judgments on whatever matters arise.
2. We must occupy ourselves only with those objects that our intellectual powers appear competent to know certainly and indubitably.
3. As regards any subject we propose to investigate, we must inquire not what other people have thought, or what we ourselves conjecture, but what we can clearly and manifestly perceive by intuition or deduce with certainty. For there is no other way of acquiring knowledge.
4. There is need of a method for finding out the truth.
5. Method consists entirely in the order and disposition of the objects towards which our mental vision must be directed if we would find out any truth. We shall comply with it exactly if we reduce involved and obscure propositions step by step to those that are simpler, and then starting with the intuitive apprehension of all those that are absolutely simple, attempt to ascend to the knowledge of all others by precisely similar steps.
6. In order to separate out what is quite simple from what is complex, and to arrange these matters methodically, we ought, in the case of every series in which we have deduced certain facts the one from the other, to notice which fact is simple, and to mark the interval, greater, less, or equal, which separates all the others from this.
7. If we wish our science to be complete, those matters which promote the end we have in view must one and all be scrutinized by a movement of thought which is continuous and nowhere interrupted; they must also be included in an enumeration which is both adequate and methodical.
8. If in the matters to be examined we come to a step in the series of which our understanding is not sufficiently well able to have an intuitive cognition, we must stop short there. We must make no attempt to examine what follows; thus we shall spare ourselves superfluous labour.
9. We ought to give the whole of our attention to the most insignificant and most easily mastered facts, and remain a long time in contemplation of them until we are accustomed to behold the truth clearly and distinctly.
10. In order that it may acquire sagacity the mind should be exercised in pursuing just those inquiries of which the solution has already been found by others; and it ought to traverse in a systematic way even the most trifling of men’s inventions though those ought to be preferred in which order is explained or implied.
11. If, after we have recognized intuitively a number of simple truths, we wish to draw any inference from them, it is useful to run them over in a continuous and uninterrupted act of thought, to reflect upon their relations to one another, and to grasp together distinctly a number of these propositions so far as is possible at the same time. For this is a way of making our knowledge much more certain, and of greatly increasing the power of the mind.
12. Finally we ought to employ all the help of understanding, imagination, sense and memory, first for the purpose of having a distinct intuition of simple propositions; partly also in order to compare the propositions.
13. If we perfectly understand a problem we must abstract it from every superfluous conception, reduce it to its simplest terms and, by means of an enumeration, divide it up into the smallest possible parts.
14. The problem should be re-expressed in terms of the real extension of bodies and should be pictured in our imagination entirely by means of bare figures. Thus it will be perceived much more distinctly by our intellect.
15. It is generally helpful if we draw these figures and display them before our external senses. In this way it will be easier for us to keep our mind alert.
16. As for things which do not require the immediate attention of the mind, however necessary they may be for the conclusion, it is better to represent them by very concise symbols rather than by complete figures. It will thus be impossible for our memory to go wrong, and our mind will not be distracted by having to retain these while it is taken up with deducing other matters.

He’d planned a further 15 but did not finish the work. These 21 were published posthumously in 1701.

It is time to bury the nonsense of the ‘incomplete animal.’ As Julian Huxley, the eminent British biologist, once observed concerning human toughness, man is the only creature that can walk twenty miles, run two miles, swim a river, and then climb a tree. Physiologically, he has one of the toughest bodies known; no other species could survive weeks of exposure on the open sea, or in deserts, or the Arctic. Man’s superior exploits are not evidence of cultural inventions: clothing on a giraffe will not allow it to survive in Antarctica, and neither shade nor shoes will help a salamander in the Sahara. I am not speaking of living in those places permanently, but simply as a measure of the durability of men under stress.

— Paul Shepard, The Tender Carnivore and the Sacred Game, 1973