#2 Measure Your Calibre

Algebra Level 5

x 2016 + ( 2016 ! + 1 ! ) x 2015 + ( 2015 ! + 2 ! ) x 2014 + + ( 1 ! + 2016 ! ) = 0 x^{2016} + (2016!+1!)x^{2015} + (2015!+2!)x^{2014} + \cdots + (1!+2016!) = 0

Find the number of integer solutions to the equation above.

Notation: ! ! is the factorial notation. For example, 8 ! = 1 × 2 × 3 × × 8 8! = 1\times2\times3\times\cdots\times8 .


Other problems: Check your Calibre


The answer is 0.

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2 solutions

Shourya Pandey
Mar 13, 2017

Suppose an integer solution existed, say a a . Note that for any integer x x , the given expression has the coefficients of x 2014 , x 2013 , . . . , x 1 x^{2014} , x^{2013} , ... , x^{1} even. So a 2016 + ( 2016 ! + 1 ! ) a 2015 + . . . + ( 2 ! + 2015 ! ) a + ( 1 ! + 2016 ! ) a 2016 + a 2015 + 1 ( m o d 2 ) a^{2016} + (2016!+1!)a^{2015} + ... + (2!+ 2015!)a + (1!+ 2016!) \equiv a^{2016} + a^{2015} + 1 \pmod{2} . If a a were even, then

0 = a 2016 + ( 2016 ! + 1 ! ) a 2015 + . . . + ( 2 ! + 2015 ! ) a + ( 1 ! + 2016 ! ) a 2016 + a 2015 + 1 ( m o d 2 ) 0 + 0 + 1 ( m o d 2 ) 1 ( m o d 2 ) 0= a^{2016} + (2016!+1!)a^{2015} + ... + (2!+ 2015!)a + (1!+ 2016!) \equiv a^{2016} + a^{2015} + 1 \pmod{2} \equiv 0+0+1 \pmod{2} \equiv 1 \pmod{2} , which is not true.

Thus a a is odd. But then 0 = a 2016 + ( 2016 ! + 1 ! ) a 2015 + . . . + ( 2 ! + 2015 ! ) a + ( 1 ! + 2016 ! ) a 2016 + a 2015 + 1 ( m o d 2 ) 1 + 1 + 1 ( m o d 2 ) 1 ( m o d 2 ) 0 = a^{2016} + (2016!+1!)a^{2015} + ... + (2!+ 2015!)a + (1!+ 2016!) \equiv a^{2016} + a^{2015} + 1 \pmod{2} \equiv 1+ 1+ 1 \pmod{2} \equiv 1 \pmod{2} , which is a contradiction.

Hence our assumption is incorrect, and no solution exists in integers.

Aaghaz Mahajan
Apr 26, 2018

Simply use RRT!!! A nice question btw @Md Zuhair Yeh bhi original hai kya??

i didn't think of rrt....amazing!!!!

Vilakshan Gupta - 3 years, 1 month ago

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