Large Exponents

Let x x be defined as follows: x = 1 2013 + 2 2013 + 3 2013 + + 201 4 2013 x = 1^{2013} + 2^{2013} + 3^{2013} + \ldots + 2014^{2013}

And for every i i , let Z i Z_i be the smallest non-negative integer such that: x Z i ( m o d i ) x \equiv Z_i \pmod{i}

Calculate the value of 2 Z 2013 + Z 2014 + 5 Z 2015 100 Z 2016 2 \cdot Z_{2013} + Z_{2014} + 5 \cdot Z_{2015} - 100 \cdot Z_{2016}


The answer is 909.

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1 solution

Ben Frankel
Feb 21, 2014

We will calculate Z i Z_i one by one for the required values, using the same trick to reduce the problem each time.

1 2013 + 2 2013 + 3 2013 + + 201 4 2013 Z 2013 ( m o d 2013 ) 1^{2013} + 2^{2013} + 3^{2013} + \dots + 2014^{2013} \equiv Z_{2013} \pmod{2013}

Grouping terms:

( 1 2013 + 201 2 2013 ) + ( 2 2013 + 201 1 2013 ) + + 201 3 2013 + 201 4 2013 (1^{2013} + 2012^{2013}) + (2^{2013} + 2011^{2013}) + \dots + 2013^{2013} + 2014^{2013}

Which factors as,

( 1 + 2012 ) ( ) + ( 2 + 2011 ) ( ) + ( 3 + 2010 ) ( ) + + 201 3 2013 + 201 4 2013 (1 + 2012)(\dots) + (2+2011)(\dots) + (3 + 2010)(\dots) + \dots + 2013^{2013} + 2014^{2013}

Every pair is divisible by 2013 2013 and thus is 0 ( m o d 2013 ) \equiv 0 \pmod{2013} , and we are left with 201 3 2013 + 201 4 2013 0 2013 + 1 2013 = 1 ( m o d 2013 ) 2013^{2013} + 2014^{2013} \equiv 0^{2013} + 1^{2013} = 1 \pmod{2013} , so Z 2013 = 1 Z_{2013} = 1 .

The same trick reduces Z 2014 Z_{2014} to 100 7 2013 + 201 4 2013 100 7 2013 + 0 2013 = 100 7 2013 ( m o d 2014 ) 1007^{2013} + 2014^{2013} \equiv 1007^{2013} + 0^{2013} = 1007^{2013} \pmod{2014} , and then we calculate 100 7 2013 mod 2014 1007^{2013} \textrm{mod}\:2014 .

2 100 7 2013 0 ( m o d 2014 ) 100 7 2013 0 or 1007 ( m o d 2014 ) 2 \cdot 1007^{2013} \equiv 0 \pmod{2014} \:\:\Rightarrow\:\: 1007^{2013} \equiv 0 \:\textrm{or}\: 1007 \!\pmod{2014}

Clearly, 2014 2014 does not divide 1007 1007 , so 100 7 2013 1007 ( m o d 2014 ) 1007^{2013} \equiv 1007 \pmod{2014} , and thus Z 2014 = 1007 Z_{2014} = 1007 .

Continuing the same ideas, we find Z 2015 = 0 Z_{2015} = 0 , and Z 2016 = 1 Z_{2016} = 1 .

2 Z 2013 + Z 2014 + 5 Z 2015 100 Z 2016 = 2 + 1007 + 0 100 = 909 2Z_{2013} + Z_{2014} + 5Z_{2015} - 100Z_{2016} = 2 + 1007 + 0 - 100 = \boxed{909}

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