Set's size belong it

Let N N be the number of subsets of { 1 ; 2 ; ; 2015 } \{1; 2; \ldots; 2015\} that contains its own size.

Find the 4 last digits of N N ?

Details and assumptions:

  • The set { 1 ; 2 ; 3 } \{1; 2; 3\} will be counted because it has 3 3 elements and contain 3 3 .


The answer is 6384.

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1 solution

For each k { 0 ; 1 ; 2 ; ; 2015 } k\in\{0; 1; 2; \ldots; 2015\} , we want to find how many elements there are of size k k .

Note that k 0 k\ne0 since 0 { 1 ; 2 ; ; 2015 } 0\notin\{1; 2; \ldots; 2015\} .

Every subset of size k k must contain k k . The other k 1 k-1 elements can be anything. Therefore, there are ( 2014 k 1 ) \displaystyle \binom{2014}{k-1} subsets of size k k that contains k k .

Therefore, N = k = 1 2015 ( 2014 k 1 ) = k = 0 2014 ( 2014 k ) = 2 2014 \displaystyle N=\sum_{k=1}^{2015}\binom{2014}{k-1}=\sum_{k=0}^{2014}\binom{2014}{k}=2^{2014} .

We have ϕ ( 625 ) = 500 \phi(625)=500 , implies 2 500 1 ( m o d 625 ) 2^{500}\equiv 1\pmod{625}

Thus, 2 2014 2 14 16384 134 ( m o d 625 ) 2^{2014}\equiv 2^{14}\equiv 16384\equiv 134 \pmod{625} .

Combining with 2 2014 0 ( m o d 16 ) 2^{2014}\equiv 0 \pmod{16} , yielding 2 2014 6384 ( m o d 10000 ) 2^{2014}\equiv 6384\pmod{10000}

So, the answer is 6384 \boxed{6384} .

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