A manuscript of calculations by Albert Einstein sold at auction yesterday in a lot that totaled at $14.98 million (13.38 million euros). The famous physicist and his longtime friend and collaborator, Swiss engineer Michele Besso, wrote the papers in 1913 and 1914.

The manuscript comprises 54 pages of calculations; 26 pages by Einstein, 25 by Besso, and three by both of them. Besso and Einstein had met at a concert over 10 years earlier; they were fast friends. According to Christie’s, which ran the auction, in 1904 Einstein helped Besso get a job at the patent office he famously worked at, and Besso also assisted Einstein on his 1905 work, four breakthrough papers that addressed a number of physical phenomena including the mass-energy equivalence (which you probably know as the source of E=mc^2).

Einstein’s penmanship is neat and orderly in the papers, with bars drawn horizontally to separate calculations. It also has a bit of personality; bits of the manuscript are crossed off and torn out. According to ABC News, in one place where an equation correctly described the relativity of rotational movements, Einstein scrawled “Stimmt!” (“It works!”) The entire artifact, which was purchased anonymously over the phone, can be viewed here.

Einstein won his Nobel Prize in Physics a century ago this year, and the price paid for his and Besso’s math shows the fascination we still have with the esteemed scientist. Einstein’s work paved the way for modern astrophysical discoveries, like the 2015 detection of gravitational waves, which Einstein predicted.

Because Einstein attained cult status during his lifetime, manuscripts like this one ended up being saved. This is perhaps the reason we were able to read a never-before-published missive about bees by the physicist this year.

Some of the recently auctioned calculations were incorrect, though. Einstein misstated the mass of the Sun in one place, an error that carried through some of the other calculations in the manuscript. Though the figure was later recalculated, some of the work remained marred by the mistake.

Perhaps that makes the manuscript even worthier, though: It’s a testament to the great difficulty of the work. A eureka moment in physics is just the visible firework that emerges from the diligent efforts of the pyrotechnicians.

More: Albert Einstein Ponders Birds and Bees in Previously Unpublished Letter