3+3+3=2;6+3=2,这是笑話嗎?不,这是完全真实的一个真理。确实有这样一些算术:6+3=2;3×4=5;3÷4=6,这些奇怪的等式,是在所谓“残数算术”中出現的。应用这个算术我們能够很快地根据已知数算出过200年后或是300年前的某一天是星期几。在这个算术中的残数可以看成是一个特别的数列。在普通算术中,自然数1,2,3,……有无限多个,它們是用来数有順序的东西的。例如,数书的頁数;铁路的公里牌;紀元年次(1960,1961,……);排队的报数等等。但是要数站成一个圓圈的七个人却不是用的 3+3+3=2; 6+3=2. Is this a joke? No, this is a completely true truth. There are indeed some arithmetics: 6+3=2; 3×4=5; 3÷4=6. These strange equations appear in so-called “residue arithmetic”. Using this arithmetic we can quickly calculate from the known number the day of the week after 200 years or 300 years ago. The residual in this arithmetic can be seen as a special series. In ordinary arithmetic, the natural numbers 1,2,3,... have infinite numbers, and they are used to order things. For example, the number of pages of a number of books; the kilometers of railways; the number of years (1960, 1961,...); the number of lines waiting in line. But seven people who are standing in a circle are not used