673 is the second prime to divide more than one number of the form p# - 1 (673 divides both 149# - 1 and 193# - 1).
673 divides 3333333331. [Benney]
The Louvre Pyramid is made up of 673 glass lozenges and triangles, not counting the doors.
Chlorophyll a fluoresces at 673 nanometers.
The smallest prime adjacent to any 3-perfect number. [Loungrides]
673 = 1(mod 6) = 1(mod 7)= 1(mod3). [Bergot]
There are 673 numbers N with no two same digits, having the
property that when the sum of the k leftmost digits of N is
divided by k the result is always an integer. [Post and
Professor David Broadhurst's favorite titanic prime is zeta(-673)/zeta(-1).
The only non-titanic prime of the form 337*2^n-1.
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