**The 21-digit resolution to the decades-old drawback suggests many extra options exist.**

What do you do after fixing the reply to life, the universe, and the whole lot? In the event you’re mathematicians Drew Sutherland and Andy Booker, you go for the more durable drawback.

In 2019, Booker, at the College of Bristol, and Sutherland, principal analysis scientist at MIT, have been the first to search out the reply to 42. The quantity has popular culture significance as the fictional reply to “the final query of life, the universe, and the whole lot,” as Douglas Adams famously penned in his novel “The Hitchhiker’s Information to the Galaxy.” The query that begets 42, a minimum of in the novel, is frustratingly, hilariously unknown.

In arithmetic, solely by coincidence, there exists a polynomial equation for which the reply, 42, had equally eluded mathematicians for a long time. The equation x^{3}+y^{3}+z^{3}=okay is called the sum of cubes drawback. Whereas seemingly simple, the equation turns into exponentially troublesome to resolve when framed as a “Diophantine equation” — an issue that stipulates that, for any worth of okay, the values for x, y, and z should every be integers.

When the sum of cubes equation is framed on this approach, for sure values of okay, the integer options for x, y, and z can develop to monumental numbers. The quantity house that mathematicians should search throughout for these numbers is bigger nonetheless, requiring intricate and large computations.

Over the years, mathematicians had managed by way of varied means to resolve the equation, both discovering an answer or figuring out {that a} resolution should not exist, for each worth of okay between 1 and 100 — besides for 42.

In September 2019, Booker and Sutherland, harnessing the mixed energy of half 1,000,000 dwelling computer systems round the world, for the first time found a solution to 42. The broadly reported breakthrough spurred the staff to sort out an excellent more durable, and in some methods extra common drawback: discovering the subsequent resolution for 3.

Booker and Sutherland have now revealed the options for 42 and three, together with a number of different numbers higher than 100, lately in the *Proceedings of the Nationwide Academy of Sciences*.

The primary two options for the equation *x*^{3}+*y*^{3}+*z*^{3 }= 3 is perhaps apparent to any highschool algebra pupil, the place x, y, and z will be both 1, 1, and 1, or 4, 4, and -5. Discovering a 3rd resolution, nevertheless, has stumped professional quantity theorists for a long time, and in 1953 the puzzle prompted pioneering mathematician Louis Mordell to ask the query: Is it even potential to know whether or not different options for 3 exist?

“This was kind of like Mordell throwing down the gauntlet,” says Sutherland. “The curiosity in fixing this query isn’t a lot for the explicit resolution, however to raised perceive how arduous these equations are to resolve. It’s a benchmark towards which we are able to measure ourselves.”

As a long time glided by with no new options for 3, many started to consider there have been none to be discovered. However quickly after discovering the reply to 42, Booker and Sutherland’s methodology, in a surprisingly quick time, turned up the subsequent resolution for 3:

569936821221962380720^{3} + (−569936821113563493509)^{3} + (−472715493453327032)^{3} = 3

The invention was a direct reply to Mordell’s query: Sure, it’s potential to search out the subsequent resolution to three, and what’s extra, right here is that resolution. And maybe extra universally, the resolution, involving gigantic, 21-digit numbers that weren’t potential to sift out till now, means that there are extra options on the market, for 3, and different values of okay.

“There had been some critical doubt in the mathematical and computational communities, as a result of [Mordell’s question] could be very arduous to check,” Sutherland says. “The numbers get so massive so quick. You’re by no means going to search out greater than the first few options. However what I can say is, having discovered this one resolution, I’m satisfied there are infinitely many extra on the market.”

To search out the options for each 42 and three, the staff began with an present algorithm, or a twisting of the sum of cubes equation right into a kind they believed could be extra manageable to resolve:

*okay* − *z*^{3} = *x*^{3} + *y*^{3} = (*x* + *y*)(*x*^{2} − *xy* + *y*^{2})

This strategy was first proposed by mathematician Roger Heath-Brown, who conjectured that there needs to be infinitely many options for each appropriate okay. The staff additional modified the algorithm by representing x+y as a single parameter, d. They then lowered the equation by dividing either side by d and retaining solely the the rest — an operation in arithmetic termed “modulo d” — leaving a simplified illustration of the drawback.

“Now you can assume of okay as a dice root of z, modulo d,” Sutherland explains. “So think about working in a system of arithmetic the place you solely care about the the rest modulo d, and we’re attempting to compute a dice root of okay.”

With this sleeker model of the equation, the researchers would solely have to look for values of d and z that might assure discovering the final options to x, y, and z, for okay=3. However nonetheless, the house of numbers that they must search by way of could be infinitely massive.

So, the researchers optimized the algorithm by utilizing mathematical “sieving” methods to dramatically minimize down the house of potential options for d.

“This entails some pretty superior quantity idea, utilizing the construction of what we learn about quantity fields to keep away from trying in locations we don’t have to look,” Sutherland says.

The staff additionally developed methods to effectively break up the algorithm’s search into lots of of hundreds of parallel processing streams. If the algorithm have been run on only one laptop, it might have taken lots of of years to discover a resolution to okay=3. By dividing the job into thousands and thousands of smaller duties, every independently run on a separate laptop, the staff may additional velocity up their search.

In September 2019, the researchers put their plan in play by way of Charity Engine, a undertaking that may be downloaded as a free app by any private laptop, and which is designed to harness any spare dwelling computing energy to collectively clear up arduous mathematical issues. At the time, Charity Engine’s grid comprised over 400,000 computer systems round the world, and Booker and Sutherland have been capable of run their algorithm on the community as a check of Charity Engine’s new software program platform.

“For every laptop in the community, they’re informed, ‘your job is to look for d’s whose prime issue falls inside this vary, topic to another circumstances,’” Sutherland says. “And we had to determine the right way to divide the job up into roughly 4 million duties that might every take about three hours for a pc to finish.”

In a short time, the world grid returned the very first resolution to okay=42, and simply two weeks later, the researchers confirmed they’d discovered the third resolution for okay=3 — a milestone that they marked, partially, by printing the equation on t-shirts.

The truth that a 3rd resolution to okay=3 exists means that Heath-Brown’s authentic conjecture was proper and that there are infinitely extra options past this latest one. Heath-Brown additionally predicts the house between options will develop exponentially, together with their searches. As an illustration, slightly than the third resolution’s 21-digit values, the fourth resolution for x, y, and z will seemingly contain numbers with a mind-boggling 28 digits.

“The quantity of work it’s a must to do for every new resolution grows by an element of greater than 10 million, so the subsequent resolution for 3 will want 10 million instances 400,000 computer systems to search out, and there’s no assure that’s even sufficient,” Sutherland says. “I don’t know if we’ll ever know the fourth resolution. However I do consider it’s on the market.”

Reference: “On a query of Mordell” by Andrew R. Booker and Andrew V. Sutherland, 10 March 2021, *Proceedings of the Nationwide Academy of Sciences*.

DOI: 10.1073/pnas.2022377118

This analysis was supported, partially, by the Simons Basis.

[Editor’s Note (March 23, 2021): A reference to “whole numbers” in the article was replaced with “integers” as negative values are allowed.]