11,213

Number

$11 \, 213$ (eleven thousand, two hundred and thirteen) is:

The index of the $23$rd Mersenne prime after $2$, $3$, $5$, $7$, $13$, $\ldots$, $2203$, $2281$, $3217$, $4253$, $4423$, $9689$, $9941$:

$M_{11 \, 213} = 2^{11 \, 213} - 1 \approx 2 \cdotp 814 \times 10^{3375}$

The Mersenne number $M_{11 \, 213} = 2^{11 \, 213} - 1$ was demonstrated to be a Mersenne prime by Donald Bruce Gillies on $2$ June $1963$ at the University of Illinois.

The University celebrated this event by using a special postmark to frank its letters.