Babylonian Pythagorean Triples

  An example of Pythagorean triples from 1800 BC Babylon can be found on the Plimpton 322 cuneiform clay tablet.

Sexagesimal notation is used on the tablet with multiples of ten written sideways and the numbers 1-9 written vertically as counts. The last column on the right is a line count and we see 1=|, 2=||, and 3=|||. Four is written in two rows with two impressions on each row, etc. 10=<, 20=<<, and 30=<<<. Forty is written in two rows with three impressions diagonally on one row and another on the second diagonal row. Zeroes are represented as empty spaces as they would be in a place notation. Two of the Pythagorean triples are in the 2nd and 3rd columns. The 2nd column is one side of the triangle and the 3rd column contains the diagonal. Neugebauer gives a translation and interpretation of the tablet as triangles The 1st column appears to be the square of the ratio of the two sides. We're missing a lot of context associated with the tablet but Robson argues the likelihood that the tablet was made by a bureaucrat favors the interpretation that the tablet represents a list of solutions to a set of similar math problems.

There are some discrepancies in the first column for lines 1, 4, and 13.

On the 1st and 13th lines Neugebauer appears to have misread spaces. The 8th line may contain a calculation error or perhaps transcription errors by a copyist.

Except for the 8th row the numbers in the 1st column are the squares of the ratio of the two sides and are accurate to 15 decimal places.

We can label the sides of the triangle as shown in the figure below.

The sexagesimals on the clay tablet can be transcribed into decimal notation as follows using the values from Wikipedia.

We can compute the missing side using the Pythagorean theorem and we find that the square of the ratio of the two sides matches the sexagesimal values exactly.

We can compare the angles of the triangles along a unit circle and the last column above shows that the deviation from 45° ranges from approximately 0-15°.

Could the list be the ratios of the sides of a set of standard triangles associated with slopes? The Babylonians were more likely to think of angles in terms of triangles and would measure a slope as a displacement x per unit change in height like we find with the Egyptian seked. The angles above range between 45° and 60°.

Supplemental (Feb 3): A skeptic on Plimpton 322. The loss of context for the tablet makes it more difficult to interpret. It may be incomplete since column could be missing on the left. Could it have been forgery? Could it be carbon dated if it contained straw or biological material? Can we rule out a practical joker? Another way we can supply context is to compare the tablet with other mathematical tablets to see if it fits in with them. What would a Babylonian administrator need such a tablet for?