The concept of square-free coloring was introduced in graph theory by Alon in 2002.

无正方形着色的概念由阿龙在2002年引入图论。

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The hardware and app are free, but Square takes a small cut of each transaction.

硬件和应用程序是免费的，但Square对每一笔交易收取一点点费用。

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And the panels have to stay dust-free: just a seventh of an ounce of dust per square yard of panel can decrease solar power conversion by 40 percent.

偏偏太阳能板需要一个无尘的环境：每平方码只要有七分之一盎司的尘土就会降低太阳能板40%的转换效率。

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Ten workers painted the 130, 000-square-foot factory for free, after-hours and on weekends.

FORBES: Sell the Sizzle

Snap Fitness gyms are small--only 2, 500 square feet--and frills free.

FORBES: Magazine Article

Under measures approved by local authorities, swathes of outdoor public places including beaches, municipal parks and even Times Square have become tobacco-free.

BBC: New York's outdoor smoking ban: Will the world follow?