# Some 2016 fun

It’s 2016! This is awesome. I wanted to create a repository of awesome facts on the number 2016. Anyone who has any nontrivial facts about the number can comment!

• Obviously, we start off with $\#\mathrm{GL}_2(\mathbf{Z}/7\mathbf{Z})=2016$, as in our previous post.
• Relevant to my project under Professor Elkies: it’s a congruent number!
• $1+2+...+63=\frac{63\times 64}{2}=2016$.
• $2016={64\choose 2}$. It’s the 63rd triangular number. It’s also hexagonal (because $63=2\times 32 - 1$, it’s the 32nd hexagonal number).
• Scott Kominers tells me that 2016 is also icositetragonal. So it’s expressible in the form $\frac{1}{2}(22n^2 - 20n)$. But honestly speaking, “icositetragonal” is a really cool name for a number. If I was a (human) icositetragonal number, I’d be a happy baby potato.
• You can prime factorize 2016 as $2^5\times 3^2\times 7$, so there are exactly fourteen abelian groups of order 2016 (where we’re utilizing the Chinese remainder theorem).
• It’s divisible by the number of its divisors.
• $\frac{1}{10}\times \#A_8 = \frac{1}{10}\times \#\mathrm{GL}_4(\mathbf{Z}/2\mathbf{Z})/10 = 2016$. The cool thing is that $A_8\cong \mathrm{GL}_4(\mathbf{Z}/2\mathbf{Z})$. I’d like to write up a proof of this someday.
• From Facebook: “The 64 primes from 79 to 439 can be put into an 8×8 magic square so that each row, column and diagonal sums to 2016:

439  89   83  97  419 379 113 397
137 149 433 317 373 163 193 251
331 349 199 179 313 271 223 151
239 167 227 233 269 307 401 173
257 353 191 263 229 103 283 337
293 281 347 367 131 277 109 211
241 197 127 421 101 157 383 389
79 431 409 139 181 359 311 107″

• Any more? Comment below!