A figurate number of the form giving the area of the square gnomon obtained by removing a square of side from a square of side ,
(1)
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(2)
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The gnomonic numbers are therefore equivalent to the odd numbers, and the first few are 1, 3, 5, 7, 9, 11, ... (OEIS A005408). The generating function for the gnomonic numbers is
(3)
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