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Gnomon (figure)
From Wikipedia, the free encyclopedia
In geometry, a gnomon is a plane figure formed by removing a similar parallelogram from a corner of a larger parallelogram.
Figurate numbers were a concern of Pythagorean geometry, since Pythagoras is credited with initiating them, and the notion that these numbers are generated from a gnomon or basic unit. The gnomon is the piece which needs to be added to a figurate number to transform it to the next bigger one.
For example, the gnomon of the square number is the odd number, of the general form 2n + 1, n = 1, 2, 3, ... . The square of size 8 composed of gnomons looks like this:
8 7 7 7 7 7 7 7
8 7 6 6 6 6 6 6
8 7 6 5 5 5 5 5
8 7 6 5 4 4 4 4
8 7 6 5 4 3 3 3
8 7 6 5 4 3 2 2
8 7 6 5 4 3 2 1
To transform from the n-square (the square of size n) to the (n + 1)-square, one adjoins 2n + 1 elements: one to the end of each row (n elements), one to the end of each column (n elements), and a single one to the corner. For example, when transforming the 7-square to the 8-square, we add 15 elements; these adjunctions are the 8s in the above figure.
Note that this gnomonic technique also provides a proof that the sum of the first n odd numbers is n2; the figure illustrates 1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 = 64 = 82.