在自然数的茫茫大海中，有一些关系密切的数，它们彼此的全部的约数和（自身除外）与另一方相等，我们把这样的数叫做“相亲数”。

220与284就是一对相亲数。把220的全部约数（自身除外）相加是：

1+2+4+5+10+11+20+22+44+55+110＝284。

同样，把284的全部约数（除掉284本身）相加是：

1+2+4+71+142＝220。

大数学家欧拉在1750年曾经一口气向公众宣布了60对相亲数，使人们大开眼界。

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