"Here is an email I wrote to my students in the Fall of 2013 that went viral, and was read by more than 1 million people." That's a very strong statement for a

"Here is an email I wrote to my students in the Fall of 2013 that went viral, and was read by more than 1 million people." That's a very strong statement for a mathematician to make. I wonder how he can prove that 1 million people "read" it.