The 3×3 problem was presented to me as a party game. Participants were told that the magic number was 15, and asked to construct the square. My method of solving the problem was by writing out the possible sums without regard to the order of their components (there are only 8) and noting that the central element must appear in 4 sums, and the corner elements in 3. 5 is the only number that appears in four sums, 2, 4, 6 and 8 appear in three, and 1, 3, 7 and 9 in only 2. Thus the only possible solution (together with its variants resulting from the symmetry of the square, which can be produced by rotation, reflection and inversion of the elements) is as below

8 1 6

3 5 7

4 9 2

No magic square exists for n = 2 but squares do exist for all n > 2 and the number of possibilities increase very rapidly with n. It happens, however, that all squares of a given n have the same magic number, given by

M = (n^{2} +1)n/2

Methods for constructing magic squares for any given n > 2, but without demonstration of their validity for all n, are given on page 148 et seq. of *Mathematical Recreations* by Maurice Kraitchik (1942). Kraitchik’s book can be read for free and without breach of copyright online (google the author, title and ‘free ebook’). The purpose of the pdf is to validate the methods Kraitchik describes.

Steiner Chains were briefly discussed in Martin Gardner’s 1979 Scientific American article and they are a generalisation of Pappus Chains (or to be more accurate Pappus Chains are a limiting case of Steiner Chains).

A distorted annulus is formed by two circles, one within the other. A Steiner Chain is an infinite sequence of circles tangent (externally) to the circles forming the distorted annulus and to each other, using a particular inscribed circle as the first in the sequence.

In certain circumstances the chain reproduces itself after one ‘circuit’ of the distorted annulus. In these cases the chain will close itself regardless of where the first circle is placed (see diagram below reproduced from Gardner’s article).

This result, referred to as ‘Steiner’s Porism’, is usually explained using inversion geometry (see for example the relevant chapter of ‘Geometry Revisited’ by H S M Coxeter and S L Greitzer).

Pappus Chains are obviously a limiting case of Steiner Chains, where the circles forming the distorted annulus are tangent. The centres of the circles forming Steiner Chains are known to lie on an ellipse (see relevant Mathworld article) so quite possibly the parameters of Steiner Chains, like those of Pappus Chains, are determined by the parameters of the ellipse. In that case, the cases when the chain closes will be determined by a diophantine equation involving those parameters.

Of course it is quite possible that if the chain does not close after one ‘circuit’ of the distorted annulus, it will close after two or more.

]]>Next I will be looking at Clifford’s Circle Theorem via his original paper, available online in his collected works, but as I will be taking a one month holiday in India and Clifford’s paper looks rather complex, I expect it will be some time before I have worked my way through it.

The India trip will include a stay in Kanchipuram, where Ramanujan spent some of his early childhood.

]]>I came across reference to Tusi while reading *Lost Enlightenment. Central Asia’s Golden Age from the Arab Conquest to Tamerlane* by S Frederic Starr. Nasir al-Din al-Tusi (1201 – 1274) was a polymath who founded what was then the world’s largest astronomic observatory at Maragha or Maragheh in what is now Azerbaijan.

What is now referred to as the Tusi Couple, and naturally was studied by Tusi, is a planar geometric construct consisting of two circles, the smaller of which is half the diameter of the larger and inside it, touching at a single point. The smaller circle rolls around the circumference of the larger without sliding in a fixed direction, anticlockwise say.

The singular feature of the Tusi Couple is that each point on the circumference of the smaller circle exhibits a periodic motion along a fixed diameter of the larger.

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Explanations of just why the method works are less frequently encountered. In this pdf, I give my take on the workings of the method.

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