Find the mininum integer n which satisfies the following property.
1) In a set including n positive but less than 1000 integers, there exists a pair of
numbers, a>b, such that every digit of a >= the corresponding digit of b.
For deion reason, you could write 89 as 089. For example:
a= 234, b=132 are ok, but a=234, b=5 is not.
应该是 a set including n positive integers。
小草MM问为什么是76,我只有一个不太满意的证明。
设有有一个集S,使得这样两个数a,b不存在,我们证明|S|<=75。设H(i)为S中百位为i的数,T(i)为S中十位为i的数。首先证明H(0)+T(9)<=11:设H(0)=i,则H(0)中有一个数各位数字分别小于等于09(10-i)。这时T(9)中最多只能有11-i个数(包括09(10-i))。类似的可以证明
H(0)+T(9)<=11
H(1)+T(8)<=13
H(2)+T(7)<=15
H(3)+T(6)<=17
H(4)+T(5)<=19
H(5)+T(4)<=19
H(6)+T(3)<=17
H(7)+T(2)<=15
H(8)+T(1)<=13
H(9)+T(0)<=11
两边加起来就有2*|S|<=150
另一半是做一个有75个数的集:只要取所有三个位数加起来为14的数就行了。