In geometry, the **spiral of Theodorus** (also called *square root spiral*, *Einstein spiral* or *Pythagorean spiral*) is a spiral composed ofcontiguous right triangles. It was first constructed by Theodorus of Cyrene.

The spiral is started with an isosceles right triangle, with each leg having unit length. Another right triangle is formed, an automedian right triangle with one leg being the hypotenuse of the prior triangle (with length √2) and the other leg having length of 1; the length of the hypotenuse of this second triangle is √3. The process then repeats; the *i*th triangle in the sequence is a right triangle with side lengths √*i* and 1, and with hypotenuse √(*i* + 1).

Although all of Theodorus’ work has been lost, Plato put Theodorus into his dialogue *Theaetetus*, which tells of his work. It is assumed that Theodorus had proved that all of the square roots of non-square integers from 3 to 17 are irrational by means of the Spiral of Theodorus.^{
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Plato does not attribute the irrationality of the square root of 2 to Theodorus, because it was well known before him. Theodorus and Theaetetus split the rational numbers and irrational numbers into different categories.^{
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Each of the triangles’ hypotenuses *h _{i}* gives the square root of the corresponding natural number, with

Plato, tutored by Theodorus, questioned why Theodorus stopped at √17. The reason is commonly believed to be that the √17 hypotenuse belongs to the last triangle that does not overlap the figure.^{
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In 1958, Erich Teuffel proved that no two hypotenuses will ever coincide, regardless of how far the spiral is continued. Also, if the sides of unit length are extended into a line, they will never pass through any of the other vertices of the total figure.^{}^{
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Theodorus stopped his spiral at the triangle with a hypotenuse of √17. If the spiral is continued to infinitely many triangles, many more interesting characteristics are found.

If φ_{n} is the angle of the *n*th triangle (or spiral segment), then:

Therefore, the growth of the angle φ_{n} of the next triangle *n* is:^{
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The sum of the angles of the first *k* triangles is called the total angle φ(*k*) for the *k*th triangle, and it equals:^{
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with

The growth of the radius of the spiral at a certain triangle *n* is

The Spiral of Theodorus approximates the Archimedean spiral. Just as the distance between two windings of the Archimedean spiral equals mathematical constantpi, as the number of spins of the spiral of Theodorus approaches infinity, the distance between two consecutive windings quickly approaches π.^{
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The following is a table showing the distance of two windings of the spiral approaching pi:

Winding No.: | Calculated average winding-distance | Accuracy of average winding-distance in comparison to π |
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2 | 3.1592037 | 99.44255% |

3 | 3.1443455 | 99.91245% |

4 | 3.14428 | 99.91453% |

5 | 3.142395 | 99.97447% |

→ ∞ | → π | → 100% |

As shown, after only the fifth winding, the distance is a 99.97% accurate approximation to π.^{
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