DM4.1

• 注意divide与除以是相反的，a divides b的意思是b除以a没有余数 $$ \fbox{a is Congruent to b modulo m的意思就是a与b能够整除以m，b-a = km} $$
• Congruence就是 “同余性”
• if a = dq + r, then a->dividend, q->quotient, d->divisor, r->remainder

Modular arithmetic

$$ \fbox{if a $\equiv$ b (mod m) and c $\equiv$ d (mod m), then ac $\equiv$ bd (mod m)} $$ - （a+b）mod m = ((a mod m) + (b mod m)) mod m - ab mod m = ((a mod m)(b mod m)) mod m