Turns out the search-engine heavyweight--which was behind the anonymous, stark-white ad with black lettering--only wanted to lure the math-minded who wouldn't need Google's database in the first place, and give them a job.

The recruitment ploy certainly plays on mathematician Paul Erdos' famous quote: "A mathematician is a device for turning coffee into theorems."

In a kind of geek "Jeopardy," the billboard read:"{first 10-digit prime found in consecutive digits e}.com." The answer, 7427466391.com, would lead a puzzle-sleuth to a Web page with yet another equation to solve, with still no sign the game was hosted by Google.

Mastering that equation would lead someone to a page on Google Labs, the company's research and development department, which reads: "One thing we learned while building Google is that it's easier to find what you're looking for if it comes looking for you. What we're looking for are the best engineers in the world. And here you are.

"As you can imagine, we get many, many resumes every day, so we developed this little process to increase the signal-to-noise ratio."

Google, which is preparing for a $2.7 billion IPO later this year, is methodical about hiring people. With 1,907 employees as of March 31, the company has been rigorous about its methods since day one. One of the company's first employees, back when Google was operating out of a garage in Palo Alto, said that Sergey Brin and Larry Page, Google co-founders, had at least eight how-to books on hiring people on a nearby desk during his interview. Company recruits have also said that they have endured numerous interviews, sometimes with math or business strategy tests, before receiving an offer.

Google has also hosted programming challenges for the last couple of years as a way to spot talent. Many of its computer scientists, however, come from Stanford University's Computer Science department, according to recruits. In another clever recruiting method, Google has sponsored paid listings on its own search results pages to draw potential staff members. For example, it has on the keyword/name Udi Manber, who is the chief of .

Google, based in Mountain View, Calif., is advertising on a billboard near the Ralston exit leading to Santa Clara. It may introduce more billboards in future locations, according to a company source.