Seventy-three is the number of books in the Catholic Bible, forty-six in the Old Testament, including the deuterocanonical books, and twenty-seven in the New, against the sixty-six of the Protestant canon. As a prime that is also, with thirty-seven, tied to the gematria of Genesis 1:1, seventy-three gathers an unusual amount of arithmetical interest for its size. Its two firm associations are the value of Chokhmah, wisdom, in the sefirot and the extent of the Catholic canon, one Kabbalistic, one ecclesiastical.
Seventy-three is prime, with no factors beyond one and itself. It is a twin prime, paired with 71, a prime just two away. Because it leaves a remainder of one when divided by four, it is a Pythagorean prime, expressible as a sum of two squares: 3 squared plus 8 squared.
The genuine Pythagorean number philosophy that this modern chart borrows its name from left no writings of its own: Pythagoras and his early followers taught only by mouth, and what survives of their number theory comes instead through Nicomachus of Gerasa, whose Introduction to Arithmetic of the first century CE became the standard authority for a thousand years, and through Iamblichus, who drew on Nicomachus two centuries later; neither wrote anything resembling a name-and-birthdate numerology.