Re:maths

開始個18個三角形數是1、3、6、10、15、21、28、36、45、55、66、78、91、105、120、136、153、171
平方數也稱正方形數
最小的11個平方數為（OEIS中的數列A000290）：02 = 0


12 = 1
22 = 4
32 = 9
42 = 16
52 = 25
62 = 36
72 = 49
82 = 64
92 = 81
102 = 100







矩形數 (rectangular number)
A number that can be shown as a pattern of dots which are in the shape of a rectangle.
Such a number must have at least two factors bigger than one. Seven is not a rectangular number (factors 1 and 7), whereas ten is (factors 2 and 5)
so 1 is not a rectangular number

2006-12-22 23:13:11 補充：
1是三角形數,正方形數﹐不是矩形數？？

2006-12-22 23:13:28 補充：
1是三角形數,正方形數﹐不是矩形數