A great THEORY of NUMBER!

For each integer 1 j 2017 1\le j\le 2017 , let S j S_j denote the set of integers 0 i 2 2017 1 0\le i\le 2^{2017} - 1 such that i 2 j 1 \left\lfloor \frac{i}{2^{j-1}} \right\rfloor is an odd integer. Let P P be a polynomial such that P ( x 0 , x 1 , , x 2 2017 1 ) = 1 j 2017 ( 1 i S j x i ) . P\left(x_0, x_1, \ldots, x_{2^{2017} - 1}\right) = \prod_{1\le j\le 2017} \left(1 - \prod_{i\in S_j} x_i\right). Compute the remainder when ( x 0 , , x 2 2017 1 ) { 0 , 1 } 2 2017 P ( x 0 , , x 2 2017 1 ) \sum_{\left(x_0, \ldots, x_{2^{2017} - 1}\right)\in\{0, 1\}^{2^{2017}}} P\left(x_0, \ldots, x_{2^{2017} - 1}\right) is divided by 2017 2017 .

This was proposed by A s h w i n S a h Ashwin Sah .


The answer is 1840.

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1 solution

The set of S j S_j is made up of S j = [ 2 j 1 , 2 2 j 1 1 ] [ 3 2 j 1 , 4 2 j 1 1 ] . . . [ ( 2 2018 j 1 ) 2 j 1 , 2 2018 j 2 j 1 1 ] S_j = [2^{j-1}, 2 \cdot 2^{j-1} - 1] \cup [3 \cdot 2^{j-1}, 4 \cdot 2^{j-1} - 1 ] ... \cup [(2^{2018 - j} - 1)2^{j-1}, 2^{2018 - j} \cdot 2^{j-1} - 1] . If we let T j = i S j x i , T_j = \prod_{i \in S_j} x_i, then P ( x 0 , x 1 , . . . , x 2 2017 1 ) = 1 j 2017 ( 1 T j ) P(x_0, x_1, ..., x_{2^{2017} - 1}) = \prod_{1 \leq j \leq 2017} \left (1 - T_j \right ) . The polynomial is 1 1 if all T i T_i are 0 0 and the polynomial is 0 0 if at least one T i T_i is 1 1 . Thus, we need to count the number of permutations such that all T i T_i are zero. We can use complementary counting and PIE. Let A i A_i denote the set of permutations where T i = 1 T_i = 1 and let P P be the set of all permutations. We want to compute X = P A i + A i A j A i A j A k + . . . X = |P| - \sum |A_i| + \sum | A_i \cap A_j| - \sum |A_i \cap A_j \cap A_k| + ... Now observe that S j = 2 2016 |S_j| = 2^{2016} for all j j so A j = 2 2 2016 |A_j| = 2^{2^{2016}} .

Now let's consider A i A j |A_i \cap A_j| . WLOG i < j i < j . Let's look at S i S_i and S j S_j first. Notice that in each part of S j S_j (and each "nonpart") half of it is taken up by parts of S i S_i . Thus, S j S_j actually fills in half of the gaps. So, there will be 2 2016 + 2 2015 2^{2016} + 2^{2015} numbers here in their union. These need to be equal to 1 1 . Thus, we obtain that A i A j = 2 2 2015 |A_i \cap A_j| = 2^{2^{2015}} .

By similar reasoning, we have that A a 1 A a 2 A a 3 . . . A a k = 2 2 2017 k |A_{a_1} \cap A_{a_2} \cap A_{a_3} \cap ... \cap A_{a_k} | = 2^{2^{2017 - k}} .

So actually, we have that X = i = 0 2017 ( 1 ) k ( 2017 k ) 2 2 2017 k 2 2 2017 2 ( m o d 2017 ) X = \sum_{i = 0}^{2017} (-1)^k \binom{2017}{k} 2^{2^{2017-k}} \equiv 2^{2^{2017}} - 2 \pmod{2017} . Let's compute 2 2 2017 2^{2^{2017}} in modulo 2017 2017 . We first compute 2 2017 2^{2017} in modulo 2016 2016 . I think 2016 = 2 5 3 2 7 2016 = 2^5 \cdot 3^2 \cdot 7 .

So, let m = 2 2017 m = 2^{2017} we have

m m 0 ( m o d 32 ) \equiv 0 \pmod{32}

m m 2 ( m o d 9 ) \equiv 2 \pmod{9}

m m 2 ( m o d 7 ) \equiv 2 \pmod {7} Now we use Chinese Remainder Theorem, uh m 2 ( m o d 63 ) m \equiv 2 \pmod{63}

m 128 ( m o d 2 ) 016 m \equiv 128 \pmod 2016 . Now by Fermat's Little Theorem, we want to reduce 2 128 2^{128} in modulo 2017 2017 . Thus, the answer is that minus 2 2 to get 1840 \boxed{1840}

I don't understand what happened in first two paras... Which theorem has been applied HR.

Ayush Kumar - 3 years, 2 months ago

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