Let be the number of subsets of that contains its own size.
Find the 4 last digits of ?
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For each k ∈ { 0 ; 1 ; 2 ; … ; 2 0 1 5 } , we want to find how many elements there are of size k .
Note that k = 0 since 0 ∈ / { 1 ; 2 ; … ; 2 0 1 5 } .
Every subset of size k must contain k . The other k − 1 elements can be anything. Therefore, there are ( k − 1 2 0 1 4 ) subsets of size k that contains k .
Therefore, N = k = 1 ∑ 2 0 1 5 ( k − 1 2 0 1 4 ) = k = 0 ∑ 2 0 1 4 ( k 2 0 1 4 ) = 2 2 0 1 4 .
We have ϕ ( 6 2 5 ) = 5 0 0 , implies 2 5 0 0 ≡ 1 ( m o d 6 2 5 )
Thus, 2 2 0 1 4 ≡ 2 1 4 ≡ 1 6 3 8 4 ≡ 1 3 4 ( m o d 6 2 5 ) .
Combining with 2 2 0 1 4 ≡ 0 ( m o d 1 6 ) , yielding 2 2 0 1 4 ≡ 6 3 8 4 ( m o d 1 0 0 0 0 )
So, the answer is 6 3 8 4 .