Starting with the number 1 and moving to the right in a clockwise direction a 5 by 5 spiral is formed as follows:
21 22 23 24 25
20 7 8 9 10
19 6 1 2 11
18 5 4 3 12
17 16 15 14 13
It can be verified that the sum of the numbers on the diagonals is 101.
What is the sum of the numbers on the diagonals in a 1001 by 1001 spiral formed in the same way?
根據題意可分析:
sum初始值爲中心數值:1
最中心的數值爲1.每一圈需要在sum上累加矩陣4個角的數值。
(1) -2- (3 5 7 9) -4- (13 17 21 25) -6- (31 37 43 49)
可知 每級矩陣之間相鄰數(上級矩陣的最後一個數和當前矩陣的第一個數)相差的值爲矩陣級數之差N;
矩陣內部4個數相差的數值也爲這個值。
所以得到遞歸方程:f(n) = f(n-1)+((n+3)/4)*2;n>0 起始條件f(0)=1