The remainder of the year, corrected.

Level 2

Let N = i = 1 2014 ( 2014 C i ) 2 N=\sum_{i=1}^{2014} (^{2014}C_{i})^{2} Find the remainder when N N is divided by 53.


The answer is 52.

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1 solution

Josh Rowley
Jan 2, 2014

Firstly we can use the identity that i = 0 n ( i 2 ) = ( 2 n n ) \sum_{i=0}^n (i^2) = \dbinom{2n}{n} . However the sum in the question begins at i = 1 i=1 , so we have to subtract one. So the question now is: Find ( 4028 ! ) ÷ ( 2014 ! ) ( 2014 ! ) 1 ( m o d 53 ) (4028!) \div (2014!)(2014!) -1 \pmod{53} . Now, 2014 ÷ 53 = 38 2014 \div 53 = 38 so 5 3 76 53^{76} is the highest power of 53 which divides the denominator. 4028 ÷ 53 = 76 4028 \div 53 = 76 , however one of these multiples of 53 is 5 3 2 53^{2} so in fact 5 3 77 53^{77} is the highest power of 53 which divides the numerator. So 53 ( 4028 ! ) ÷ ( 2014 ! ) ( 2014 ! ) 53 \mid (4028!)\div(2014!)(2014!) , and therefore ( 4028 ! ) ÷ ( n ! ) ( n ! ) 1 0 1 52 ( m o d 53 ) (4028!) \div (n!)(n!) - 1 \equiv 0-1 \equiv 52 \pmod{53}

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