It's still the early 2015

What is the last 4 digits of 2 2016 { 2 }^{ 2016 } ?


The answer is 5536.

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3 solutions

Discussions for this problem are now closed

Firstly, 2 4 x 1 ( m o d 5 ) 2^{4x} \equiv 1 \pmod{5} with every positive integer x x (easily proved).

As a result:

2 20 1 = ( 2 4 1 ) ( 2 16 + 2 12 + 2 8 + 2 4 + 1 ) 2^{20}-1=(2^4-1)(2^{16}+2^{12}+2^8+2^4+1) is divisible by 25, means 2 20 1 ( m o d 25 ) 2^{20} \equiv 1 \pmod{25} ;

2 100 1 = ( 2 20 1 ) ( 2 80 + 2 60 + 2 40 + 2 20 + 1 ) 2^{100}-1=(2^{20}-1)(2^{80}+2^{60}+2^{40}+2^{20}+1) is divisible by 125, means 2 100 1 ( m o d 125 ) 2^{100} \equiv 1 \pmod{125}

Once more and we have: 2 500 1 ( m o d 625 ) 2^{500} \equiv 1 \pmod{625}

So: 2 2000 1 ( m o d 625 ) 2^{2000} \equiv 1 \pmod{625}

( 2 2000 1 ) (2^{2000}-1) is divisible by 625 -> ( 2 2004 2 4 ) (2^{2004}-2^4) is divisible by 10000

Then ( 2 2016 2 16 ) (2^{2016}-2^{16}) is divisible by 10000

The last 4 digits of 2 2016 2^{2016} is the same as that of 2 16 2^{16} , which is 5536 \boxed{5536} .

Moderator note:

You should mention that the 2 4 x 1 ( m o d 5 ) 2^{4x} \equiv 1 \pmod 5 can be proven using Fermat's Little Theorem.

Mj Santos
Jan 31, 2015

To get the last 4 digits of 2 2016 2^{2016} , we need to get the remainder when it is divided by 10000.

To get the remainder you need to use CRT or Modular Arithmetic

By Modular Arithmetic:

2^2016mod10000=2^8^252mod10000=256^252mod10000=65536^126mod10000=5536^126mod10000=30647296^63mod10000=7296^63mod10000=(1616^31)(7296)mod10000=(1456^15)(1616)(7296)mod10000=(9936^7)(1456)(1616)(7296)mod10000=(4096^3)(9936)(7296)(2896)mod10000=(7216)(7856)(9216)mod10000=(7216)(896)mod10000=5536mod10000

Therefore, the last 4 digits of 2 2016 2^{2016} is 5536 \boxed{5536 }

Moderator note:

Even though you got it right, it's better to do as little calculation as possible.

Can we use this to find last digit of any exponent ?

Parth Bhardwaj - 6 years, 3 months ago

If yes then good method

Parth Bhardwaj - 6 years, 3 months ago

Yes we can. It's simply m o d 10 \bmod {10} .

Pi Han Goh - 6 years, 2 months ago
Murali Kancharla
Jan 31, 2015

we need to divide by 10,000 and remainder would be last 4 digits. 2^2016=65536^126= (6*10000+5536)^126 = 126C0(10000)126+... last term of expansion would be 5536. Each term has atleast one 10,000. Hence remainder is 5536

Moderator note:

This solution has been marked wrong. The last term of the expansion is 553 6 126 5536^{126} , you still got quite a number of manual labor to go through this. And it is impractical to calculate or memorize 2 16 = 65536 2^{16} = 65536 .

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